Cut the tomatoes in half and gently wring out the seeds and liquid over the sink. Grate each tomato half on the coarse side of a grater into a bowl by holding the cut side of the tomato half against the grater and grating the flesh just to the skin. Discard the skin. Grate the onion into the bowl. Stir in the parsley, paprika, oil, and lemon juice, salt and black pepper to taste; the mixture should be highly seasoned. Serve immediately, or no longer than 4 hours after making, at room temperature.
Note: Fora milder harissa, omit the hot paprika or cayenne pepper.