Asparagus, Heirloom tomato and caramelized onion tart

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• 8 ounces thin asparagus, ends trimmed
• 4-6 ounces mini heirloom tomatoes, cut in half
• 1/2 large onion, thinly sliced lengthwise 
• Olive oil
• Red pepper flakes
• 8 ounces Mascarpone cheese
• 1/2 - 1 cup Gruyere cheese
• Large handful fresh basil leaves
• Zest and juice of 1 lemon
• Kosher salt 
• Freshly ground pepper
• Flour for dusting work surface
• 1 sheet of all-butter puff pastry ( I use a Dufour 14 ounce sheet if I don't have time to make from scratch)
• 1 egg beaten for egg wash

1. Heat oven to 425 degrees.
2. Drizzle olive oil into skillet over medium high heat, add onions, salt,  pepper and red pepper flakes.  Saute onions 5- 10 minutes until golden brown.  They are great this way in the tart and this is how I make it most of the time.  If you have more time and want to caramelize them, stir in some butter and brown sugar and continue to cook until they are are very deep golden brown in color.  This takes around 40 minutes.  Set cooked onions aside.
3. In your food processor, add basil, lemon zest and juice, a few teaspoons of olive oil, mascarpone cheese and dash of salt and pepper.  Process until you have a smooth paste consistency.
4. Toss the asparagus and tomatoes in a VERY small amount of olive oil - just to lightly cover them.
5. Place your puff pastry dough on a floured work surface and roll it out a few times into a 15-by 11-inch rectangle.
6. With a knife, lightly score a 1/2 inch border around the edges of the puff pastry.  Transfer the dough to a parchment lined baking sheet.
7. Spread the mascarpone mixture inside the border
8. Add the onions on top of the mascarpone.
9. Line up the asparagus on top of the onions and scatter the cut tomatoes on top.
10. Lightly salt and pepper.
11. Sprinkle with a handful or so of gruyere on top.
12. Brush egg wash on exposed pastry border.
13. Bake until the pastry is puffed and golden, about 15-20 minutes.

Let cool before cutting - Enjoy!