Prepare spinach according to package directions; drain and pat dry with paper towels. Set aside.
Remove stems from mushrooms, discarding stems.
Melt 2 tablespoons butter in a large skillet over medium-high heat. Add 12 mushrooms; sauté 4 minutes or until barely tender. Repeat procedure with remaining mushrooms and 2 tablespoons butter. Remove from skillet, and place cap-side up in a 15- x 10-inch jelly-roll pan. Place 1 oyster piece in each mushroom cap.
Melt remaining 2 tablespoons butter in a large skillet over medium-high heat. Add artichoke and onion; sauté 3 to 4 minutes or until tender. Remove from heat, and stir in reserved spinach, sour cream, 1/3 cup cheese, and salt. Spoon mixture evenly into mushroom caps; sprinkle with remaining cheese and crumbled bacon.
Bake at 350° for 20 minutes or until thoroughly heated.