Heat oven to 400.
Cover 2 sheet pans with foil. Place potatoes on one sheet, asparagus on the other. Toss each with 1 tablespoon of olive oil. Season with salt and pepper.
Place sheet pans into the oven. Cook for 25 minutes (potatoes should be fork tender, asparagus should be tender but still crisp.
Meanwhile, pat fish dry with paper towels and season with salt and pepper.
Heat 1 tbs olive oil in a large (nonstick if you have it) pan over medium-high heat, until shimmering. Cook fish until golden brown, about 5 minutes per side. Transfer fish to platter and tent loosely with foil.
Add shallot to now-empty skillet and cook until softened, about 2-3 minutes. Stir in tomatoes and cook until just softened, about 1 minute. Stir in sour cream, mustard, and capers and cook until slightly thickened, about 2 minutes. Season with salt and pepper to taste.
Top fish with sauce, sprinkle with tarragon, and serve with potatoes and asparagus.