Heat large skillet over medium heat and add 1-tablespoon butter and 1-tablespoon oil.
When the oil/butter are heated (before butter burns), add onions and cook, stirring often, until onions are softened, 4-5 minutes.
Add artichokes and cook, stirring often, 4-5 minutes or until they have a lightly browned.
Add garlic, stir and cook another 30 seconds.
Add remaining 1 tablespoon butter and stir until melted.
Sprinkle the flour over the vegetable mixture and cook 1 minute, stirring continuously.
Slowly stir in the half-and-half, Parmesan, lemon juice, hot sauce, Worcestershire Sauce and salt.
Reduce heat to low and simmer, stirring occasionally, 3 minutes or until thickened.
Remove from the heat, add spinach and stir well.
Spoon the artichoke/spinach filling into the mushroom caps, packing down and rounding over the tops of the mushrooms.
Top the filled mushrooms with the topping mixture, pressing down lightly so the topping sticks to the filling.
Bake 20-25 at 350°F or until the tops are golden brown and mushrooms are cooked through.
Remove from oven, let rest 4 minutes and serve.