Preheat the oven to 425 degrees.
Cut the summer squash and zucchini 1/4-slices and bell peppers into 1″ slices.
In a small bowl combine olive oil, mustard, apple cider vinegar, garlic, salt, and pepper.
Place all the cut vegetables in a single layer on 2 baking sheets. Drizzle vegetable with olive oil and mustard dressing.
Bake for 25 to 35 minutes, until all the vegetables are tender, turning once with a metal spatula. Or, place on a hot grill on indirect heat. Cook on each side for 1-2 minutes until soft.
Remove from oven or grill.
Drizzle with balsamic vinegar. Sprinkle with feta cheese and parsley.
Season to taste. This dish can be served hot or cold.
This recipe reheats well.