Preheat the broiler.
Sprinkle the mushrooms with salt, then fry in the butter and 2 tablespoons olive oil over medium-high heat until lightly golden, adding in the balsamic vinegar for the last 2 to 3 minutes. Add the sugar; taste and add more as needed. Remove and set aside while you prepare the bread.
Slice the baguette 1/2-inch thick on the diagonal. Lay the slices flat on a baking sheet and brush with 2 teaspoons olive oil. Broil until lightly golden on both sides, 2 to 3 minutes per side.
Meanwhile, roughly mix together the ricotta, yogurt, parsley, lemon zest and some olive oil. Season with salt and pepper.
To assemble, spread each bruschetta with some of the ricotta mixture, top with a spoonful of the mushrooms, add an extra drizzle of olive oil and some lemon juice if you like. Serve immediately.