01 -
In a medium skillet, melt butter with olive oil over medium heat. Add onion, season with salt and pepper, and cook, stirring occasionally, until softened and golden brown, about 10 minutes.
02 -
Set oven rack to the middle position and preheat to 425 °F (220 °C).
03 -
Using a round cookie cutter, cut circles from puff pastry sheets and arrange them evenly on a parchment-lined baking sheet.
04 -
Whisk the egg together with water. Brush the surface of each pastry round generously with egg wash.
05 -
Top each pastry round with a slice of tomato. Add a spoonful of caramelized onion over the tomato, then sprinkle each with shredded pepper jack cheese.
06 -
Bake for 16 to 20 minutes or until the pastry is golden, puffed and the cheese is melted and beginning to brown.
07 -
Remove tarts from oven, transfer to a wire rack, garnish with fresh parsley and herbs. Cool briefly before serving.