Marry two Italian comfort-food classics to make this delicious gnocchi-mac mashup.
In medium skillet, cook leeks and thyme in 2 tbsp. butter over medium-high, stirring often, until softened, about 10 minutes. Transfer to plate. In same skillet, add remaining 2 tbsp. butter and flour. Cook, stirring often, until pale yellow, about 2 minutes. Add milk. Whisk until thickened, about 8 minutes. Add 1 1/2 cups cheese and mustard. Stir until cheese melts; season. Stir in leeks, peas, and gnocchi. Top with remaining 1/4 cup cheese. Broil until bubbly, about 5 minutes.