Prepare potatoes according to package instructions.

Meanwhile, fill a large bowl with cool water. Add 1 teaspoon salt and lemon juice. Place morels in the bowl for about 3 to 4 minutes, pushing them down into the water several times to clean them. Drain on paper towels. Gently pat dry; let stand to dry while continuing recipe.

Melt 1 tablespoon butter in a small skillet over low heat. Add onions, 1/4 teaspoon salt, and 1/8 teaspoon pepper; sauté until soft and translucent, about 10 minutes.

Preheat oven to 350°. Fill a pastry bag with 1/4-inch round tip with mashed potatoes. Insert the tip into the opening at the top of a morel, and pipe potato mixture in until morel is full. Transfer to a baking sheet. Repeat process with remaining potatoes and morels. (Stuffed morels can be prepared up to this point 1 day in advance and stored, covered, in the refrigerator.)

In a small saucepan (or in the microwave), melt remaining 2 tablespoons butter over medium-low heat. Brush tops of morels with butter, and transfer to the oven. Bake until morels are tender and potatoes are hot and slightly golden, about 20 minutes if morels are baked right away and 30 minutes if prepared the day before. Serve immediately.

Serves 6 to 8