Wash the collard greens in several changes of water. Remove the stems. Stack the greens a few at a time and cut crosswise into 1/2-inch wide strips. Bring a large pot of water to the boil, add the collard greens and cook 10 minutes. Drain and rinse with cold water. Squeeze out the excess moisture.
Heat the oil in a large skillet. Add the onion and jalapeno; saute 5 minutes. Add the tomatoes and cook 1 minute.
Stir in the greens, salt and pepper. Cook 5 minutes. Stir in the lemon juice and cook 1 minute. Serve.