Preheat your oven to 350 degrees F. Slice baguette evenly into 4 slices. Slice vertically across each top and hollow each slice out, pulling out the bread inside. Set aside.
In a medium bowl, whisk red wine vinegar, Worcestershire sauce, mustard, garlic and olive oil.
Place mushrooms and red onion slices in a large baking dish and pour marinade over. Stir and/or toss to coat mushrooms and red onions evenly. Season with salt and pepper and set aside.
Transfer mushrooms and red onions in their marinade to a large skillet over medium-high heat (keep baking dish to transfer mushroom/onion mixture back into). Cook over medium-high heat for 5 to 6 minutes, stirring constantly, or until mushrooms are tender.
Transfer mushrooms and onions back into marinade dish. Add fresh chopped spinach and stir well to combine and coat spinach as evenly as possible. Spinach will wilt while your baguette is heating.
Place baguette slices onto a baking sheet and bake for approximately 5 minutes. Remove from oven and carefully stuff each baguette piece with mixture and top with shredded cheese. Serve immediately.