Pre-heat oven to 375 degrees F. Saute the onion and garlic in 2 teaspoons of olive oil for 5-6 minutes until slightly translucent.
Place in the bottom of a round casserole dish or tart pan.
Slice the potatoes, squash and zucchini in round ¼ inch thick slices. I used my mandolin, it made perfect slices!
In a large mixing bowl stir together the vegetables, olive oil, Italian seasoning, oregano, salt, pepper, and Parmesan cheese. Stir until veggies are coated.
Layer them alternatively in a circular motion on top of the onion mixture. Make sure to fit them in tightly and create a spiral. Sprinkle with more cheese and bake at 375 for 30-40 minutes, or until golden brown.