Center the oven rack; preheat the oven to 425°F. Cut the brocoli florets and the bell pepper into rough half-inch pieces, and slice the onion into thin half-moons. Put the veggies on a small baking sheet, and give them a light drizzle of olive oil and a sprinkling of salt and pepper. Toss to coat. Roast the veggies in the preheated oven until they soften and color just slightly -- 18-20 minutes. Set aside to cool. Do not turn off the oven!
Whisk the egg, Parmesan (or Asiago), and seasonings in a medium bowl. Then whisk in the mascarpone. Whisk until the mixture becomes fairly smooth. Set aside.
On a parchment-lined baking sheet, scatter the Swiss cheese in a rough 13x10-inch rectangle. On a non-floured work surface, roll the puff pastry dough into a (rough) 13x10-inch rectangle. Transfer the pastry to the baking sheet, laying it directly on top of the cheese. Using a sharp knife, score a 3/4-inch border all along the edge of the dough.
Spread the mascarpone mixture evenly over the dough, staying within the scored border. Scatter the vegetables over the mascarpone in a single layer. Sprinkle on the optional sliced scallion and thyme leaves.
Bake in the preheated oven until the pastry puffs and colors and the mascarpone is cooked through -- about 25 minutes. Cool on the baking sheet for 5 minutes, then slide the tart -- still on its parchment paper -- onto a wire rack. Let cool for 10 minutes before slicing into 8 portions.
*You will need just 3 1/2 - 4 cups of cut up vegetables for this tart.