Step 1Heat the oven to 375 degrees. Coat the inside of a 9-inch tart pan with a removable bottom with the cooking spray.
Step 2Fit the piecrust into the pan and fold the excess crust under to reinforce the sides of the tart. Prick the bottom of the crust with a fork. Bake for 10 minutes. Remove the crust from the oven and brush the bottom with the mustard. Sprinkle with the chopped thyme and set aside.
Step 3Stir together the grilled vegetables, Gruyere, basil and salt and pepper to taste.
Step 4Spread the vegetable mixture evenly in the crust. Starting at the outside edge, place the tomato slices on top, overlapping the slices and alternating the colors until you have reached the center. Sprinkle the tomatoes with the Parmesan.
Step 5Bake until the crust is golden brown and the cheese mixture is bubbling, 40 to 50 minutes. Slice and serve warm or at room temperature.