Boil sweet potatoes in slightly salted water, drain and add a tablespoon of butter. Press it through a ricer. Heat oil on a pan and fry finely diced onion with crushed garlic. Add 1 tablespoon of tomato concentrate, herbs de Provence and some salt and pepper to taste. Put meat in a baking dish, mixing it thoroughly with a spoon. Top with sweet potato puree and bake in the oven for 10-15 minutes.
Tip: You may replace sweet potatoes with regular potatoes, for example if there are any left after dinner.