Preheat the oven to 400F and line a sheetpan with parchment paper for easy clean-up
Snap off the ends of the asparagus, which can be tough or bitter. Add the fish, tomatoes, and asparagus to the sheetpan, leaving some room between everything so it cooks evenly.
Drizzle the olive oil, juice from 1/2 lemon, thyme, and capers over the fish and veggies and bake for 12-15 minutes. Use a fork to flake the fish to make sure it is cooked through. Serve with lemon slices.