In a large dutch oven, heat the olive oil over medium heat. Once it's hot, add the diced onion and minced garlic. Toss around with a wooden spoon and cook until onions are softened and slightly browned, about 3-5 minutes. Be careful as the garlic can easily burn.Once the onions are cooked, add the Italian sausage and ground beef. Cook until it's no longer pink, breaking it up with a wooden spoon.Once meat is browned, drain any juices in the pot. Return to heat and pour in the crushed tomatoes and tomato sauce. add a little water to the jars, shake a bit and pour into the pot.Add tomato paste, sugar, salt, pepper, dried basil, Italian Seasoning, fennel seeds and red pepper flakes. Stir to make sure everything is combined. Turn the heat to low and simmer for at least 1 and ½ to 2 hours to reduce the water and enhance the flavour. Skim the fat on top and stir occasionally, tasting the sauce and adjusting the seasonings to your preference. Use for lasagna, spaghetti and more!