Preheat your oven to 350°F (175°C) and lightly coat a 9x13-inch baking dish with cooking spray.

In a large pot of boiling, salted water, cook the jumbo shells according to package instructions, but reduce cooking time by one minute. Drain and set aside.

In a medium saucepan, melt 2 tablespoons of unsalted butter over medium heat. Add diced onion and sliced mushrooms, sauté until tender, about 5 minutes. Remove from heat.

In the same saucepan, add 1 tablespoon of butter and heat over medium-low. Whisk in 2 tablespoons of flour, stirring constantly for about 2 minutes.

Gradually pour in 1 cup of skim milk and 1 cup of half-and-half, whisk continuously until fully combined and slightly thickened. Cook for 5 minutes.

In a small bowl, temper egg yolks by whisking in a few spoonfuls of hot cream sauce. Stir back into saucepan off heat.

In a large bowl, combine sautéed vegetables, seafood, parsley, and half of the prepared cream sauce. Stir until well coated.

Fill each jumbo shell with the seafood mixture and place them in the baking dish.

Pour the remaining cream sauce over the stuffed shells and sprinkle grated Parmesan cheese on top. Bake for 10 minutes or until cheese is melted and bubbly.