Heat large skillet or sauté pan over medium-high heat. Add bacon and cook until browned and crispy. Drain on paper towels; set aside.
Reserve 2 tablespoons of bacon grease in pan and return to heat. Add onions and pinch of salt and pepper. Sauté onions 5 minutes, stirring regularly. Add 1 tablespoon water, stirring to deglaze pan; stir in any browned bits. Remove onions to bowl.
Return pan to heat and add 1 tablespoon butter and 2 tablespoons water. Bring to simmer and add spinach and pinch of salt and pepper. Cook for 3-5 minutes to wilt the spinach. Remove spinach to a bowl.
Heat a grill or grill pan over medium-high heat. Split whole-grain baguettes. Assemble sandwiches: On each baguette bottom, spread 1-2 tablespoons jam, 2 ounces Limburger slices, spinach leaves, onions, 2 slices of bacon, and baguette top, in that order. Carefully turn the sandwiches over and butter the bottoms. Place sandwiches, bottom-side down on hot grill, then butter the tops of the baguettes. When bottom sides are golden and cheese starts to melt, carefully flip sandwiches and brown sandwiches' tops, melting the cheese.
- 8 thick slices bacon
- 1 yellow onion, thinly sliced
- Salt and pepper
- 2-3 tablespoons water, divided
- 8 tablespoons butter, divided
- 10-12 ounces fresh spinach leaves
- 4 whole-grain baguettes
- 4-8 tablespoons fig jam or preserves
- 8 ounces Wisconsin Limburger cheese, sliced