Wring thawed spinach in towels until moisture is removed, reserve.
In a skillet, sauté onion in butter until tender. Dust onion with flour, add in milk and bring to a boil. Stir in Parmesan cheese and thawed spinach. Season according to taste.
Slice baguette into 1/2-inch slices. Butter each side, slather with spinach mixture, top with Colby & cheddar slices.
Pan fry on each side until golden.