In a large saute pan, melt butter on medium high heat. Add shallots and garlic; cook for 30 seconds. Then add mushrooms to the pan. Season with salt and pepper to taste and cook for 8-10 minutes or until mushrooms are lightly golden. If mushrooms begin to stick to the pan, add butter or olive oil as needed.To the mushrooms, add half and half (it should sizzle), then stir in the baby spinach. Remove from heat. Taste for salt and pepper.To assemble crostini: top crostini with piece of burrata cheese and then with warm mushroom mixture. Sprinkle with red pepper flakes (optional). Serve immediately.Preheat oven to 350 degrees. Arrange baguette slices on a large baking sheet; drizzle both sides with oil, and lightly season with salt and pepper.Bake until golden, 8-10 minutes per side. Let cool on baking sheet.