Cut the muffins in half, lightly toast, then spread with a little margarine and yeast extract, if using.
Divide the tomato slices between the muffins and place a slice of cheese on top. Place under a hot grill until just beginning to melt.
Meanwhile pour about 2.5 cm ( 1in) water into a large frying pan and bring to simmering point. Break each egg into the water and poach over a medium heat until the eggs are cooked. Lift out with a slotted spoon place an egg on each muffin. Serve at once.