In a large pot, melt the butter on medium. Add the onions, mushrooms, garlic, and thyme. Season with salt. Cook 5 min., stirring often.Sprinkle the flour over the mushrooms. Cook 1 min., stirring often. Stir in the milk and broth. Heat to a boil on high.Reduce heat to simmer. Cook 6 min., until liquid has thickened, stirring occasionally. Stir in the spinach until wilted. Divide among 4 bowls and top with croutons.