Wash and dry the collard greens, then slice them into thin ribbons. Heat the oil in a skillet and cook the onion for 6 minutes until softened.
Add the tomatoes and garlic and cook for 4 minutes. Add the greens, salt, pepper, and water. Cover for 3 minutes, then uncover and cook for 6 to 8 minutes until tender but still green.
Make the Ugali:
Bring the water and salt to a boil in a heavy saucepan. Whisk in one-third of the maize meal gradually until smooth.
Reduce the heat to low. Add the remaining maize meal in small additions, stirring firmly with a wooden spoon after each, until the ugali is smooth, very thick, and pulls from the pan, about 10 minutes.
Serve:
Cover the ugali and rest it for 3 minutes. Turn it onto a plate or shape portions with a damp spoon.
Serve immediately with the warm sukuma wiki.
Notes
Different maize meals absorb water differently; add a little extra hot water if the ugali firms before the meal is fully cooked.
Do not overcook the greens; a little texture keeps the dish fresh and appealing.