Preheat the oven’s broiler and set the oven rack about 6 inches from the heat source.
Whisk the olive oil, vinegar, dill, salt, and pepper together in a mixing bowl until evenly blended. Add the onion, zucchini, yellow squash, and radishes; toss until evenly coated. Spoon into an 8×8-inch baking dish, and cover with aluminum foil.
Broil in the preheated oven 10 minutes, then remove the foil, and continue broiling until the vegetables are tender and nicely browned on top, about 10 minutes more.