1. Cut the tops off the bell peppers and remove the seeds and membranes. Set them aside. In a pan, heat the olive oil over medium heat. Add the chopped onions and minced garlic to the pan. Sauté until they become translucent. Add the finely chopped mushrooms and cook until they release their moisture and start to brown, which should take about five minutes. Stir in the chopped spinach and cook until it wilts, approximately two minutes. Add the butter and lemon juice and stir in. Transfer the mushroom and spinach mixture to a large mixing bowl. Add the cooked rice and shredded cheese. Season with salt and black pepper to taste. Mix well to combine. Carefully stuff each bell pepper with the mushroom, spinach, rice and cheese mixture.

2. In your slow cooker, pour in the canned crushed tomatoes, diced onion, minced garlic, balsamic vinegar and vegetable stock. Season with salt and pepper. Place the stuffed bell peppers into the slow cooker on top of the tomato mixture. Cover and cook on low for two to three hours, or until the bell peppers are tender. Serve the stuffed bell peppers with the tomato and tarragon sauce, sprinkled with some soft herbs to garnish.