Caramelized Onion, Bacon, and Blue Cheese Tart

4 slices hickory smoked bacon, cut crosswise into 1/2 in. pieces
2 medium sweet onions, thinly sliced
freshly ground pepper
1 sheet frozen puff pastry, thawed
2 oz. crumbled blue cheese (1/2 C.)

Heat oven to 400 degrees. Line a baking sheet with parchment paper.
Cook the bacon in a large skillet over medium-high heat, stirring often, until crisp, 6-8 min.; transfer to a paper towel lined plate. Spoon off and discard all but 2 Tbsp. of the bacon drippings from skillet.
Add onions to the skillet and cook, stirring often, until very tender and caramelized, 12-15 min. Add a splash of water to the skillet if necessary to help release drippings. Stir in the bacon and 1/4 tsp. pepper.
Lay the pastry flat on the prepared baking sheet. Spread the onion mixture over the pastry, leaving a 1/2 in. border; spread blue cheese over the top.
Bake until the pastry is golden and crisp along the edges, 25-28 min. Transfer to a cutting board and slice into squares. Serve warm or at room temperature.