Vegetable Tian: Classic Gratin Comfort

extra-virgin olive oil -- plus more for preparing the pan

small sweet yellow onion -- diced, about 1 cup

small red bell pepper -- diced, about 1 cup

garlic -- minced

medium zucchini (about 8 ounces) -- unpeeled, sliced 1/8-inch thick

medium yellow squash (about 8 ounces) -- unpeeled, sliced 1/8-inch thick

small Yukon Gold potatoes (about 10 ounces) -- unpeeled, sliced 1/8-inch thick

small, thin sweet potato (about 10 ounces) -- unpeeled, sliced 1/8-inch thick

small tomatoes (about 12 ounces) -- sliced 1/4-inch thick

small butternut squash -- peeled, sliced 1/8-inch thick

sea salt

ground black pepper

dried thyme

dried rosemary -- crushed

shredded cheese -- Gruyere, Fontina, or Gouda (see notes)