Coat a nonstick skillet with cooking spray and heat over medium-low heat. Add shallots and minced garlic; cook 2 minutes, stirring occasionally. Increase heat to medium-high and add mushrooms; cook until wilted and still a little moist, about 3 to 4 minutes. Off heat, stir in walnuts, salt and pepper.
Rub each baguette slice with the halved garlic and then top with 2 tablespoons of mushrooms; sprinkle with sage. Serve immediately. Yields 1 per serving.