An easy apricot-mustard sauce both seasons the pork and dresses the salad.
Preheat oven to 425 degrees F.
Combine 1/2 cup vinegar, 1/2 cup water, and 1/2 tsp. salt in a medium heatproof glass bowl or mason jar. Stir until the salt dissolves. Place onion in a colander set over the sink. Pour boiling water over the onions to wilt them slightly. Add the onions to the vinegar mixture, pressing to submerge them. Cover with a lid or plastic wrap and set aside.
Combine apricot preserves and mustard in a small bowl. Place 1 Tbsp. of the mixture in a large bowl for the dressing.
Heat 1 Tbsp. oil in a large ovenproof skillet over medium-high heat. Season pork with the remaining 1/8 tsp. salt and pepper and add to the pan. Cook until browned on all sides, 5 to 7 minutes. Carefully spread 1 Tbsp. of the apricot mixture over the pork. Transfer the pan to the oven and roast until an instant-read thermometer inserted in the thickest part of the pork registers 145 degrees F, 13 to 15 minutes. Transfer to a cutting board and let rest for 5 minutes.
Place the pan (careful, the handle will be hot) over medium heat. Add 1 Tbsp. vinegar; cook for 2 minutes, stirring and scraping up any browned bits. Remove from heat; add the remaining apricot mixture and stir to combine. Thin with a little water, if the sauce is too thick.
Add garlic and the remaining 1 Tbsp. each vinegar and oil to the reserved apricot mixture in the large bowl; whisk until smooth. Add spinach and tomatoes; toss to coat. Divide the salad among 4 plates. Drain the pickled onions and distribute over the salads.
Slice the pork and arrange atop or beside the salads. Drizzle the pork with the pan sauce.
To make ahead: Refrigerate pickled onions (Step 2) for up to 1 day.