• Heat olive oil in a skillet, add the onion and sauté until transparent. Add the garlic and sauté an additional 30 seconds. Add the puréed tomatoes and bring to a boil, reduce heat and simmer covered for 10 minutes. Set aside.
• Melt the butter in a large saucepan. Add the zucchini, summer squash and corn and cook for 2 minutes. Add the water; cover and cook for 8 minutes or until the zucchini is crisp-tender. Drain and set aside.
• In a skillet heat 1/2-inch of oil over medium heat. Add the tortillas one at a time and cook about 30 seconds per side. Reserve.
• Preheat oven to 375 degrees. Butter a large casserole dish.

To assemble...

• Place a thin layer of vegetable layer on bottom. Top with a layer of 4-5 tortillas. Then add a layer of tomato sauce. Add half of the remaining vegetables, half of the poblano strips, 1/2 cup of the sour cream in dollops, half of the chicken and 3/4 cup of shredded cheese. Repeat layers finishing with the cheese.
• Bake uncovered until the cheese has melted and the layers are heated through, about 15 - 20 minutes.