1. Melt butter in a medium skillet over medium heat. Add onions and cook, stirring occasionally, until soft and golden, 8 to 10 minutes.
2. Add maple syrup and water and continue to cook, stirring occasionally, until onions are deeply browned and sticky, 12 to 15 more minutes. Season with salt and pepper to taste, stir in vinegar, and cook until mixture is the consistency of jam, 3 to 5 minutes; set aside.
3. Heat oven to 400 degrees. Unroll one pie crust onto a lightly floured surface. Using a 1 3/4" biscuit cutter, cut out 24 circles. Press each circle into a greased mini-muffin tin and pierce bottoms with a fork.
4. Grease the bottom of a second mini-muffin tin and carefully place on top of the dough-lined tin. Transfer stacked tins to oven and bake 5 minutes. Remove top tin and continue to bake until edges of dough are just golden, 2 to 3 more minutes. Transfer to a wire rack and repeat with second pie crust.
5. Increase oven to 425 degrees. Combine Gruyere, mascarpone, thyme, and 1/2 teaspoon pepper in a small bowl. Arrange tart shells on a baking sheet and place 1/2 teaspoon cheese mixture evenly in each shell. Top cheese mixture with 1/2 teaspoon caramelized onions and bake until cheese mixture is melted and bubbling, about 5 minutes. Garnish with more thyme and serve warm.
Source: Hannaford fresh Magazine, November - December 2017