In a medium skillet, heat olive oil to medium low heat. Add sausage and onions and cook until sausage is no longer pink and onions are translucent (approximately 13-15 minutes).
Line bottom of the casserole dish with bread slices (about half of baguette). Scoop sausage mixture from pan and sprinkle evenly over bread (discard extra oil in pan). Place tomatoes evenly over sausage mixture. Sprinkle with ⅔ of the shredded cheese and cover with bread slices (You may have a small amount of bread remaining. Reserve for another use). Sprinkle remaining cheese over the bread.
In a large bowl or measuring cup, whisk together cream, half and half, eggs, rosemary, and pepper. Slowly pour egg mixture over bread, working to cover each top piece of bread with egg mixture.
Cover with plastic wrap and refrigerate for 30 minutes (or overnight).*
Meanwhile, preheat oven to 350˚F. Bake for 40 minutes or until browned and cooked through (it should reach a minimum internal temperature of 160℉). Allow to rest for 10 minutes before serving.
Notes
*If refrigerated overnight, remove breakfast strata from the refrigerator while the oven is preheating.