Wash and remove the stems from the mushrooms. Optionally, dice the stems and add to the stuffing. Scrape most of the black gills from the caps with a spoon, trying not to break them. Discard the gills.
Remove the skin from the sausage and cut into 1/4 to 1/2 inch dice.
Peel and mince the garlic or roughly chop the roasted garlic.
Remove the seeds and membranes from the bell pepper. Slice off 4 strips for decorating the mushrooms. Chop the remaining pepper into 1/4 inch dice.
Microwave the cheese wedges and milk for 1 minute in a microwave safe cup. Stir to make a sauce. Don't worry about lumps.
Mix the sausage, leeks, diced bell pepper, garlic, and sauce in a large bowl. Mound 1/2 of this mixture on each mushroom cap.
Assembly: Place the mushroom caps, gill side up, in an oven save dish.
Sprinkle 1/2 the cheese over each mushroom. Lay red pepper strips across the top to decorate.
Bake at 350F for 20 minutes. Increase the oven temperature to 450F and cook an addition 10 to 15 minutes until hot and bubbly.