Shrimp Stroganoff

1/4 cup minced onion
5 tablespoons butter, divided
1 1/2 pounds shelled, raw shrimp
1/2 pound fresh mushrooms, quartered
1 tablespoon flour
1 1/2 cups sour cream (at room temperature)
1 1/4 teaspoons salt
Pepper, to taste
Cooked yellow rice
Artichoke hearts, quartered

In a large skillet saut