- Heat the oven to 325 degrees F.
- Thinly slice the bottom of the celery sticks so they will lie flat for filling. Place prepared sticks side-by-side on a baking sheet. Set aside.
- To make the filling: In a large frying pan, crumble and cook the sausage until browned and cooked, about 8 to 10 minutes. Remove the pan from heat, drain, and set aside the sausage to cool. Once cooled, add the sausage, mushroom, green onions, and garlic cloves to a food processor bowl (alternatively, you can use a blender) and pulse in a 2-second burst until well blended. Add the Boursin cheese and pulse until well blended. Scrape the mixture into a large bowl, and add the Panko crumbs, 1/3 cup of the Parmesan cheese, and 2 tablespoons of minced parsley. Mix and fold until well combined. Set aside.
- To make the topping: In a small bowl, melt 2 tablespoons of butter in a microwave-safe dish. Add the remaining 2 tablespoons of Panko crumbs and Parmesan cheese and mix until well combined. Stir in the remaining tablespoon of parsley leaves and red pepper flakes. Set aside.
- Transfer the filling into a pastry bag and pipe it onto prepared celery sticks (alternately, you can fill it with a small spatula). Sprinkle topping, leaving 1/3 of the topping mixture to garnish once baked.
- Transfer assembled celery boats to the oven and bake for 30 minutes until the top is lightly golden. Remove from the oven. Garnish with remaining topping and serve immediately.