Preheat the oven to 400°F.
Wash the mushrooms well, removing the stems. Chop the stems, set aside. Add the mushroom caps, hole side up, to a baking dish (or dishes).
Heat the olive oil in a pan over medium-high heat. Add the chopped mushroom stems and onions, cooking until onions and mushroom pieces have softened. Stir in the salt and pepper.
Stir in the garlic, then add the spinach, cooking until the spinach has softened and wilted, stir in ¼ cup beer and the artichokes.
Add the cream cheese, stirring until it has melted and combined with the rest of the filling ingredients. Stir in the cheese until melted.
Add heaping spoonfuls into the holes in the mushrooms.
Stir together the melted butter and breadcrumbs. Add a teaspoon or so to the top of each mushroom.
Add the beer to the pan around the mushrooms (if using two pans, divide the beer between the pans).
Cover the pan and bake for 18 minutes, remove the cover and cook for an additional 10 minutes or until the top has browned.