This extraordinary bite-sized appetizer has three easy, make-ahead components that are assembled just before serving. The salty and sweet combination is delicious. If you want a special hors d’oeuvre for a brunch, try these tartlets.
Combine the flour and salt in a food processor fitted with the metal chopping blade and pulse to combine. Add the butter and cream cheese and pulse about 10 times, until the mixture begins to clump together. Gather up the dough and shape into a thick disk. Wrap in plastic wrap and refrigerate until chilled and easy to handle, about 2 hours.
Preheat oven to 400ºF. Position oven rack in the center of the oven. Butter two 12-cup miniature muffin pans. Divide the dough into 24 balls. One at a time, place a ball of dough in a muffin cup, and use your fingers to press it firmly and evenly up the sides to make a pastry shell. (A wooden tart tamper, available at kitchenware stores and online, can help the job go quickly.) Pierce the shells with a fork. Freeze for 5 minutes.
Place in oven and bake until the shells are golden brown, about 10 to 12 minutes. (The shells will shrink during baking.) Let cool in the pans for 5 minutes. Carefully remove from the pans and cool completely. (The pastry shells can be stored in a covered container for up to 1 day.)
Keep the oven at 400°F.
Arrange the bacon in a single layer on a large rimmed baking sheet. Bake until the bacon is crisp and golden brown, 15 to 20 minutes. Carefully pour off the fat from the baking sheet. Brush half of the maple syrup over the bacon strips. Return to the oven and bake until glazed, about 2 1/2 minutes. Remove from the oven and flip the bacon over. Brush the other sides with the remaining syrup and grind the pepper on top to taste. Bake again until glazed, about 2 minutes longer. Transfer the bacon to a chopping board and let cool slightly. Cut each strip of bacon crosswise into sixths to make 24 equal pieces and let cool. (The bacon can be kept at room temperature for 2 hours.)
Heat the oil in a large frying pan over medium-high heat. Add the onions and cook, stirring occasionally, until softened, about 5 minutes. Reduce the heat to medium-low and cook, stirring occasionally, until very tender and golden brown, about 30 minutes. Season with salt and pepper. (The onions can be prepared 4 hours before serving. Reheat over low heat.)
Spoon warm onions into tart shells.Top each with 2 or 3 blackberry pieces, and stand the bacon between the berries. Sprinkle with the sage and serve.