Seed, core, and finely chop tomatoes. Measure 3 1/2 cups chopped tomatoes; place in a 6- to 8-quart stainless-steel, enamel, or nonstick heavy pot. Bring to boiling, stirring occasionally; reduce heat. Simmer, covered, for 10 minutes. Measure 3 1/3 cups tomatoes; return to pot. Stir in lemon juice and basil.