Preheat oven to 375 degrees F. Spray a 9 x 13 baking dish or 2 1/2 quart baking dish with nonstick cooking spray.
In a skillet, fry up bacon until crispy. Remove with a slotted spoon to a paper-towel lined plate, leaving 2 Tablespoons of the bacon grease in the skillet. Discard the excess grease, if needed.
Prepare asparagus by removing woody ends, and cutting into 2-inch pieces.
In a large bowl, mix asparagus, mayo and shredded cheddar cheese.
Add diced onions, and garlic to the bacon grease, and sauté until onions are soft.
Add the flour to the onions, and stir to make a roux.
Over medium low heat, slowly add the milk, whisking constantly until the white sauce is thickened. Season with salt and black pepper to taste.
Add the white sauce to the asparagus mixture.
Pour into the prepared dish.
Top with reserved bacon bits and French Fried Onions.
Bake for 35 - 40 minutes until bubbly and golden brown. Do not overcook to prevent mushy asparagus.
Let set for 5 - 10 minutes before serving to let the casserole set up.