Preheat oven to 400F. Brush baguette slices on both sides with oil. Toast in oven on baking sheet until golden, about 7 min. Melt butter in a non-stick frying pan over medium-high. Add shallot, mushrooms and salt. Cook until mushrooms are soft and liquid evaporates, about 6 min. Remove from heat. Add lemon zest, juice and 3 tbsp parsley. Spread 2 tsp Boursin® on each toast. Top with mushroom mixture. Sprinkle with remaining 1 tbsp parsley.