- Brush mushroom caps generously with olive oil and season with salt and pepper.
- Place mushroom caps on a high-temperature grill and cook for 5-6 minutes on each side. After flipping, spread each cap with 2 tablespoons Boursin cheese. Allow cheese to slightly melt (approximately 1 minute) and then remove mushrooms from the grill.
- Add a generous amount of caramelized onions to the bottom buns. Then add a mushroom cap. Next add a sliced tomato, and finish with top of bun.
- Place a sauté pan over medium heat. Add the butter. When melted, add shallots and garlic. Stir often while cooking for 5 minutes. Mixture should be lightly caramelized.
- Remove pan from heat and fold in spinach, thyme, basil, salt, and white pepper. Allow to cool to room temperature.
- Using a mixer, cream together the cheeses until smooth. Using a rubber spatula, fold spinach mixture into cheese mixture until incorporated evenly. Place in a container and refrigerate for up to 1 week.
Yield 1½ pints