Step 1Bring a medium saucepan of water to a boil. Trim, clean and wash the cactus. Cut each paddle into half-inch squares. Drop the cactus pieces into the boiling water and cook for 5 minutes. Drain.
Step 2Trim the stem ends of the tomatoes and cut the tomatoes into quarters. Place them in a blender container with the onion and one-fourth cup water. Blend thoroughly.
Step 3Heat the olive oil in a saucepan. Add the tomato mixture, epazote, chicken base and salt. Bring to a boil and simmer, uncovered, 10 minutes.
Step 4Add the cactus squares and the remaining one-fourth cup water. Bring to a boil and simmer gently, uncovered, 5 minutes. If the sauce becomes too thick, thin with a little water.