Step 1

In a large skillet, heat 2 tablespoons olive oil over high heat until shimmering. Working in batches and being sure not to crowd the pan, add zucchini, season with salt, and cook, turning, until just tender and browned in spots, about 4 minutes per batch. Add more oil as needed to prevent pan from drying out, and adjust heat as needed throughout to maintain a very hot, but not heavily smoking, pan. Transfer each batch to a baking sheet and spread in an even layer to cool, then transfer cooled slices to a second baking sheet or plate. Repeat with remaining zucchini, squash, and eggplant until all vegetables are lightly browned.

Step 2

In a medium saucepan, heat 2 tablespoons olive oil over medium heat until shimmering. Add garlic and onion and cook, stirring, until softened, about 4 minutes. Add crushed tomatoes, bring to a simmer, and cook, stirring and adjusting heat to maintain simmer, for 15 minutes. Blend to smooth puree with a hand blender or in a countertop blender, then add marjoram or oregano. Season with salt and pepper.

Step 3

In an earthenware, ceramic, or glass baking dish, spoon just enough sauce to cover bottom of dish in a thin, even layer. Arrange sautéed vegetable slices in an alternating layered pattern (see note) on top of sauce until entire dish is filled. Spoon a thin layer of sauce on top of vegetables; reserve remaining sauce for another use.

Step 4

When ready to bake, preheat oven to 450°F. Bake until tian is fully heated through and lightly browned on top, about 15 minutes. Serve.