Table Inheritance In SQL

First of all I apologise if this is a dumb question. I am from a object-orientated background and am trying to convert a legacy application to have an SQL backend. My SQL knowledge is fairly basic...

I have an application where I am trying to maintain a list of recipies that are composed of base ingredients.
No problem so far two tables Ingredients and a Recipie table coupled with another table Element which maintains the many-to-many relationship.

Ingredients
ID (PK)
Name
Cost

Recipie
ID (PK)
Name

Element
ID (PK) <-- maybe not necessary
RecipieFK <-- the recipie this element belongs to
Amount <-- the amount of the ingredient used (e.g. 200g)
IngredientFK <-- the ingredient that makes up this element

My problem comes because Recipies can also form part of other recipies, for example the White Sauce is composed of
Milk (ingrediant)
Flour (ingrediant)
Butter (ingrediant)

However a lasagne is composed of
Minced Beef (ingrediant)
Tomatoes (ingrediant)
Lasagne Pasta (ingrediant)
Onions (ingrediant)
Basil (ingrediant)
White Sauce (recipie)

In a OO environment this is simple, I have a base class Ingredient and a Recipie class derives from Ingredient. Each Recipie contains a list of Ingredients that make up the dish. As Recipie derives from Ingredient that list of ingredients can include other recipies.

The best SQL I've come up with so far is changing the Element table to be
ID (PK) <-- maybe not necessary
RecipieFK <-- the recipie this element belongs to
Amount <-- the amount of the ingredient used (e.g. 200g)
IngredientFK <-- an ingredient that makes up this element (can be NULL)
ComplexIngredientFK <-- a recipie that makes up this element (can be NULL)


And to insist that one of IngredientFK or ComplexIngredientFK must be NULL and the other cannot be. This bothers me because it doesn't feel right. There's redundancy built into this design, you'd have to use tiggers to wipe one FK if the other was specified and it is crying out for a more elegant solution, but so far I have drawn a blank.

Can anyone suggest a more appropriate schema!
Thanks in advance
Paul