Heat a skillet on the stove top to medium heat. Add the oil to the pan and brown the sausage meat and onions. Add the spinach and cook until wilted. In a bowl, beat the eggs with the water or milk and seasonings. Once your meat mixture is cooked, sprinkle the diced tomatoes over the top, tear the basil leaves over everything and pour the egg mixture on top. Turn the heat down to medium-low and cover the skillet. Continue cooking until the eggs have firmed up. Cover the top of the eggs with grated cheese (if used) and pop it in the oven under the broiler until the cheese is melted and bubbly.
Allow the frittata to cool slightly and then slide the whole thing out onto a serving dish. Cut into wedges and serve. This is great as a dinner along with a big green salad with balsamic dressing.
Other suggested frittata combinations: Sweet potato, leeks and rosemary; Carrots, chicken and tarragon; Beef, broccoli and mushroom; Pork, parsnips and apples.