1. Heat oil in a medium nonstick skillet over moderately high heat until hot but not smoking. Add onion and cook, stirring, until golden brown, about 5 minutes. Remove from heat; stir in thyme and squash blossom. Transfer to a bowl and let cool to room temperature. Stir in cheese; season with salt and pepper.
2. Preheat oven to 450 degree F and arrange rack in lower third. Lightly oil a large cookie sheet. Unfold piecrust on a lightly floured surface and roll out with a floured rolling pin into a 14-inch round. Place on prepared cookie sheet and spread evenly with cheese mixture, leaving a 2-inch border around edges. Decoratively arrange zucchini and squash slices over cheese to form a pinwheel, overlapping in center; season with salt and pepper. Fold piecrust edges up and over filling, forming a 1-1/2-inch crust. Brush crust with egg wash and chill 15 minutes.
3. Bake until crust is golden brown and zucchini and squash are tender, about 20 minutes. Transfer tart on cookie sheet to a rack to cool slightly. Using a large metal spatula, carefully slide tart onto a platter, garnish with thyme sprigs, if desired, and serve immediately. (Can be made ahead. Cool, wrap in foil and chill up to 1 day. Bring to room temperature before serving or reheat in a 350 degree F oven 10 minutes.) Makes 8 servings.