I always save fat from chicken broth, making bacon, whatever, but the beef fat? It doesn't sound like a good idea to make scrambled eggs in.
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I have used leftover beef fat to roast root vegetables, and it's delish! I've also heard of people adding it to the oil they use to fry french fries for a beefy kick.
Fear not! You can use it just like any other cooking fat, but it will add rich, (dare we say) beefy undernotes to whatever you're cooking. I'm thinking your cast-iron skillet, the beef fat, sweet or new potatoes, rosemary, salt, pepper, and you're on your way to really delicious hashbrowns. Ok, so cook your eggs in butter, olive oil, etc...but there's breakfast for ya.
Yorkshire Pudding to go with a roast ....
3/4 cup all-purpose flour
1/2 teaspoon salt
3/4 cup milk
1/2 cup beef fat
Preheat the oven to 450 degrees. Beat eggs and milk and stir in four and salt. Pour fat into an 8x8 baking dish. Put the pan in oven to get the fat very hot, take the pan out of the oven and pour in the batter. Put the pan back in oven and cook until puffed and dry, 15 to 20 minutes.
I like to use different animal fats for making roux. Cook it and pour it onto something flat to cool in the fridge or freezer, cut it up into cubes, and keep it in the freezer ready to use whenever you need. I also do this with chicken fat. Then, use the appropriate roux for your recipe.
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Make roux with it. Use equal parts beef fat and flour by weight. Cook to a nice dark golden brown. Freeze in ice cube trays (I have some trays set aside that I only use for odiferous things), pop out and store in ziplock bags. Use to thicken soups, sauces, gravies.......
If I'd slowed down and read Chef Krull's answer, I wouldn't have just duplicated. His/her advice is perfect.
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