Preheat oven to 350°F (175°C).

Toast pine nuts in a dry skillet for 5 minutes, or until lightly browned. Set aside.

Steam spinach in a covered skillet for about 2 minutes, until wilted.

Rinse spinach with cold water and squeeze out excess liquid. Chop finely.

Heat a small amount of butter in a skillet over medium heat.

Sauté minced shallots in the skillet until softened.

Increase oven temperature to 375°F (190°C).

Butter a shallow baking dish.

In a bowl, mix together toasted pine nuts, chopped spinach, sautéed shallots, ricotta cheese, pesto sauce, egg yolk, and pepper.

Pound chicken breasts slightly to flatten them.

Place 1/2 slice of prosciutto on each chicken breast.

Top prosciutto with 1/6 of the spinach and ricotta filling.

Fold chicken breasts in half to enclose the filling.

Place stuffed chicken breasts in the buttered baking dish.

In a small bowl, mix melted butter and lemon juice.

Spoon butter-lemon mixture over the chicken breasts.

Bake in preheated oven for 30 minutes, or until chicken is cooked through and tender, basting occasionally with the butter-lemon mixture.

Remove chicken to a platter and keep warm.

To make the sauce, melt butter in the same skillet used for the shallots.

Sauté sliced mushrooms in butter for 3 minutes. Remove mushrooms from skillet and set aside.

Add chicken stock to the skillet and bring to a boil, reducing to 1/4 cup.

Add white wine to the skillet and boil until reduced to 1/2 cup.

Add whipping cream to the skillet and boil until reduced to 3/4 cup.

Return sautéed mushrooms to the sauce.

Add any accumulated juices from the baked chicken dish to the sauce. Bring to a boil, stirring.

Stir in chopped fresh basil.

Serve chicken breasts on a platter and pass the sauce separately.