PREHEAT. Preheat oven to 375 degrees.
CUT. Using a mandoline, thinly slice the potatoes and zucchinis into thin rounds. Remove the top and seeds of the red bell pepper and cut width-wise on its side into rings or circles, then cut each ring in half.
TOSS. In a large bowl, toss the vegetable slices with 2 tablespoons olive oil, fresh thyme, and 1/2 teaspoon each of salt, garlic powder, and fresh ground pepper. Set aside.
SKILLET. Add 2 tablespoons olive oil to a large skillet over medium-high heat. Add onions and season with salt and pepper. Cook until translucent and softened, about 5 minutes. Add garlic and continue to cook for 1-2 minutes.
OIL. Oil the bottom of a large baking pan or two 10-inch round baking dishes and spread the cooked onion garlic mixture evenly in the bottom.
BAKE. Arrange the vegetables in a overlapping pattern (as pictured) alternating potatoes, zucchini, and red pepper slices. Sprinkle with remaining thyme leaves. Cover tightly with foil and bake for 45 minutes.
REMOVE. Remove foil and sprinkle evenly with panko and vegan parmesan cheese. Return to oven and bake uncovered for an additional 25 minutes until lightly browned. Serve warm.