Add the butter and olive oil to a large pan over medium heat. When the butter melts, add the shallots, garlic and rosemary and saute until the shallots are translucent. Don't let the garlic burn. Add the mushrooms, and saute until browned and most of the liquid has evaporated.
Remove from heat. Add the lemon peel, cream and cheeses.
While the mixture is cooling, slice the bread diagonally into 1/2 inch pieces. Brush with olive oil and sprinkle with coarse salt.
Spread the cooled mixture on the crostini. Place them on a baking sheet and place under the broiler until they get bubbly.