In a small saucepan over low heat, slowly cook garlic and cream until garlic is soft enough to mash with a spoon, about 45 minutes. (Cream will be reduced and thick.) Remove from heat and mash garlic into cream with a fork, making a rough purée. Stir in salt and panko and mix thoroughly.
Preheat oven to 450°. Lay mushroom caps, top side down, on an oiled baking sheet. Brush edges with oil and fill centers with garlic cream.
Bake until starting to brown, about 15 minutes. Let sit 5 to 10 minutes. Lift mushrooms from any released liquid and arrange on plates or a platter. Sprinkle with pepper.
Note: Nutritional analysis is per serving.