Preheat the oven to 400deg F. Grease an 8-cup oval gratin dish with the 2 teaspoons of the butter and set aside.
In a large sauté pan over medium-high heat, melt the remaining 2 tablespoons of the butter. Add the onions and season lightly with salt and pepper, and sauté for 2 minutes. Add the flour and cook for 1 minute, stirring constantly.
Stir in the milk and bring the mixture to a simmer. Cook for 4 to 6 minutes or until the mixture is thick enough to coat the back of a spoon. Add the peas and mix thoroughly, then pour into the prepared pan.
In a small bowl season the breadcrumbs with Essence, then sprinkle the breadcrumbs over the peas and top with the grated cheese.
Place the pan in the oven and bake until bubbly and the top is lightly golden, about 10 to 12 minutes. Remove the pan from the oven and serve.
Makes 4 to 6 servings