The Mets began where they left off: with frustration.

The boos began just a few minutes and 26 pitches into the game. David Peterson, as has often been the case this season, could not find the strike zone.

He walked Willy Adames to load the bases with one out in a first inning that promised to be like so many innings during this Mets spiral.

But after the jeers and after a brief mound visit, relief replaced the rage.

Peterson induced an inning-ending double play, which set the tone for a game and the club hopes for a season: The Mets have dug themselves a large hole and will need to begin climbing out.

Perhaps the process began when Peterson escaped one ditch and led the Mets to a 7-2 victory over the Brewers in front of 35,295 at Citi Field on Tuesday night.

Brandon Nimmo homered twice, Francisco Lindor crushed his 200th career dinger and Daniel Vogelbach added a two-run shot to power the Mets (36-43), who finally showed some life.

“The old saying of how you eat an elephant,” Nimmo said after his first homer, in the fourth inning, marked a breakthrough for the previously hitless club. “You do it one bite at a time.”

Peterson finally resembled the valuable piece he had been before this wayward season.

The lefty, who entered play with an 8.08 ERA, had been a mess — and not just at the major league level. He was demoted in mid-May and posted a 6.00 ERA in his past five starts with Triple-A Syracuse.

But the Mets needed a starter to replace the optioned Tylor Megill, and Joey Lucchesi pitched Friday. They liked what they saw from Peterson’s development, even if the results had not yet followed.

“I knew what I threw out there my first couple of [big-league] starts, and I knew it wasn’t up to par and it wasn’t what the team needed, what I needed,” said Peterson, who broke camp with the team but never could find his stuff, particularly his slider. “It was pretty easy for me to get over that [demotion] and focus on what I needed to do to get back.”

The lefty’s slider got stronger as the game went on, and he lasted six innings on a career-high 110 pitches. He pitched around traffic (five hits and three walks) and consistently was aided by his defense in his best outing of the season.

The Mets, whose rotation has lacked length all year, improved to 22-3 when their starter goes at least six innings.

Before the game, general manager Billy Eppler offered manager Buck Showalter a vote of confidence and pinpointed the poor pitching as the biggest surprise of the chaotic first half for the most expensive team in Major League Baseball history.

Maybe Peterson heard him.

The 27-year-old got every ground ball he needed: Ten of his 11 batted-ball outs came on the ground. Seemingly whenever trouble arose, the ball went south and usually to Lindor.

No out was more spectacular than the last one Peterson recorded. With two outs in the sixth inning, Owen Miller singled on Peterson’s 102nd pitch. After a mound visit, Peterson remained in for an eight-pitch battle with Luis Urias, who grounded sharply into the shortstop hole.

Lindor sprinted, backhanded and leapt — in an effort that would make Derek Jeter proud — and threw out Urias just in time.

“Hell yeah,” Lindor said after his defense outshone his offense — even if he homered two batters after Nimmo in the fourth. “I wanted the ball to be hit to me. … I was able to finish the play. I was definitely pumped.”

Peterson’s performance, excellent defense and the offensive outburst gave the sinking Mets just their second win in their past seven games. They still have dropped 16 of their past 22 and are far removed from playoff contention, but they will hope that Monday’s punchless, series-opening loss will be their rock bottom.

“We all know what Pete’s capable of,” Showalter said. “We saw it in the past and I saw it last year, a little bit this year. But we need him and Tylor and everybody down below to be more than depth.

“Hopefully this is a springboard to him doing what he is capable of doing.”

The Mets hope it will be more than Peterson who jumps on that springboard.