In a large skillet, heat 3 tablespoons butter over medium-low heat. Add 3 coarsely chopped large onions (about 8 cups) and 1 teaspoon salt and cook, stirring occasionally, until creamy and golden brown, 35 to 40 minutes. Lower the heat to low, add 3/4 cup balsamic vinegar, 1/4 cup maple syrup and 1/4 cup water and cook, stirring occasionally, until the mixture is the consistency of jam, about 1 hour and 15 minutes. Meanwhile, preheat the broiler. In a microwavable bowl, melt 2 tablespoons butter. Slice 1 baguette into 24 diagonal slices. Brush the butter on one side of each slice. Arrange the bread buttered side up on a broiler pan; toast. Top each toast with some thinly sliced manchego cheese, a thin pear slice and a dollop of onion jam. Broil just until warmed through.