Heat the oil in a frying pan and add the sausages to the pan, fry while breaking them up into small pieces with a spatula, when the sausages are nearly cooked move them to a side to free up some space on the pan and add the chopped onion, continue cooking for 5-7 minutes, then add the minced garlic and cook for 30 seconds longer.
Now add the cooked lentils, tomato paste and chicken stock, bring to a simmer and cook for 2-3 minutes until the liquid is evaporated. Taste and salt if needed. Take off the heat and aside until later.
Preheat the oven to 180C/350F. Wash the peppers, cut the tops off and remove the cores with seeds and membranes by gently pulling and twisting them out of the peppers.
Stuff the peppers with the sausage and lentil filling.
In the baking dish mix the crushed tomatoes, chicken stock or water, sliced basil leaves, garlic powder and salt. Transfer the stuffed peppers into the baking dish and stand them up right.
Cover the baking dish with a lid or tin foil and cook in the preheated oven for 45 minutes. Then take out of the oven uncover and top each pepper with grated mozzarella, return to the oven uncovered for 12-15 minutes longer or until the cheese is bubbly and golden and the peppers are cooked all the way through. (They should look a little wrinkly when cooked.)