Heat the olive oil in a deep saucepan over medium heat. Add the onions and garlic and cook a few minutes. Stir in the pepper flakes and bay leaf; cook until the onions are translucent but do not let the garlic brown. Add the fish fumet and simmer about 15 minutes. Season to taste with salt and pepper. Add the lemon juice if you want. This base can be made hours ahead and refrigerated.