Place a baking sheet on top rack and preheat oven to 450 degrees. Wash and dry all produce. Dice potatoes into 1/2-inch pieces. Trim and quarter zucchini lengthwise; cut crosswise into 2-inch pieces.

Place potatoes in a medium pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 15-20 minutes. Reserve 1/2 cup potato cooking liquid, then drain and return potatoes to pot. Mash with sour cream and 1 TBSP butter (2 TBSP for 4 servings) until smooth, adding splashes of reserved potato cooking liquid as needed. Keep covered off heat.

While potatoes cook, toss zucchini in a large bowl with a large drizzle of oil, salt, pepper, and Italian Seasoning. Carefully spread out on preheated baking sheet. Roast on top rack until golden brown and slightly crispy, 14-16 minutes.

Pat pork dry with paper towels; season all over with salt and pepper. Heat a large drizzle of oil in a large, preferably nonstick, pan over mediumhigh heat. Add pork and cook until golden brown and cooked through, 2-3 minutes per side. (For 4 servings, you may need to cook in batches.) Transfer to a plate. Wipe out pan.

Add 1/4 cup water (1/3 cup for 4 servings), stock concentrate, and any resting juices from pork to same pan over medium heat. Stir in jam and vinegar. Bring to a simmer and cook until slightly thickened, 2-3 minutes. Turn off heat and stir in 1 TBSP butter. Season with salt and pepper.

Stir a splash more reserved potato cooking liquid into potatoes to rewarm, if necessary. Divide potatoes, zucchini, and pork between plates. Spoon sauce over pork and serve.