If it is not already ready, cook some sweet potatoes in the steam, in the oven, in water or microwave and remove the pulp until you have about 400 g. Chop the onion very finely and fry with a little oil in a frying pan until transparent. Add the lentils dry, brown for a couple of minutes and cover with double its volume of water adding a pinch of salt.
Or simply cover just enough with water without puddling them and cook gently, adding more water little by little if they need it so as not to stick to the bottom, until they are very tender and the liquid has evaporated (about 15-20 minutes). Remove and let cool.
Apart, mix the cooked or roasted pulp of the crushed or crushed sweet potato, the oat flakes, the ground almonds, one or two heaping teaspoons of the mixture of spices that you prefer and the breadcrumbs. Add the lentils, mix and season. Cover and refrigerate at least an hour.
Preheat the oven to 180ºC with air or heat up and down. Line a large tray with parchment paper and go forming small balls the size of a walnut with your hands. We will smudge a bit but it shouldn’t be excessively sticky; we can grease their hands with oil.
Brush the meatballs with olive oil using a kitchen brush or spray, so that they are more golden and tasty, and bake them for about 20 minutes. Serve with the sauce mixing the yogurt with the tahini, the lemon juice, the mustard and a little salt and pepper.