- Make the filling: Though the squash and garlic are used separately, it’s best to roast them together. Preheat oven to 400 degrees. Peel squash, remove seeds, and slice evenly. Place on sheet tray in a single layer. Drizzle with olive oil, and toss with thyme, salt, and pepper. Cut garlic bulb in half, drizzle with olive oil, and wrap tightly in aluminum foil. Place on sheet tray with squash. Roast until caramelized, about 30-40 minutes. Let cool before assembling tart.
- Make the ricotta: In a large saucepan, heat milk until it reaches 190 degrees, stir to keep from scorching. Remove from heat and add lemon juice. Let sit undisturbed for 5 minutes so the curds separate from the whey. Line a colander with three layers of cheesecloth and place over a large bowl. Pour curds and whey through cloth, reserving whey for bread or other recipes if you wish. Let strain for an hour: it should be soft and creamy. Add salt and roasted garlic to the ricotta and mash with a fork until combined. Refrigerate until ready to use. Can be made up to 3 days in advance.
- Assemble tart: Preheat oven to 375 degrees. Roll out dough into the shape of your tart pan, then lay pastry over pan and press into corners. Cut around the edge, with about 1/2 inch of dough hanging over the sides. Chill in freezer for 30 minutes (or keep for up to 1 month, well-wrapped). Blind bake pastry, by laying parchment over dough and using dried beans or pie weights, for 20 minutes until edges are puffed and slightly golden. Remove weights, poke all over with a fork, and bake uncovered for an additional 10 to 15 minutes until bottom is baked through, but not dark. Let cool slightly.
- Spread 1 1/2 cups of roasted garlic ricotta on bottom of tart. Arrange roasted squash over ricotta and sprinkle thyme leaves, salt, and pepper over top. Bake for 30 minutes until ricotta has puffed up slightly. The edge of the tart should be golden brown. Let tart cool and remove from pan before serving.