Add the oil to a pan over medium heat. Once the oil is hot add the onion and cook for a few minutes until they start to soften. Add the garlic, ginger, turmeric, garam masala and cumin. Stir and cook for minute to cook the spices. Add the tomato, potato, lentils and bay leaf, mix well. Add 3/4 cups water, stir. Partially cover and simmer for about 20 minutes, until the potatoes are tender and the sauce is thickened and there is no more liquid. Turn off the heat. Use a fork to mash the potatoes and some of the lentils, leaving some whole. Add salt.(Note: Salt is never added to lentils before cooking because they will take longer to cook).
Divide the dough into 4 equal parts. Roll each into a ball. Take 1 ball (cover the rest so they don’t dry out) and roll out to a 6-inch (15 cm) circle. You should not need any flour for this. Cut the circle in half.
Take one of the halves and make a cone by taking the two ends of the cut side folding them into the center of the curve so the center of the cut side makes the point of the curve. Wet the edges with water using your finger and press the edges firmly together making sure they are well sealed. Don’t be afraid to manipulate the dough and shape. If you mess up, reshape the dough and start again.
Place the 'cone' between your thumb and first finger. Fill with 1 tablespoon of the lentil mix. Wet the open edge and pinch together well to seal and fold twist on itself to finish the triangle shape. You can see a demo of this in the video. Repeat until all the samosa are made.
Heat the 2 cups of oil in a high sided pan to 350°F/176°C or when the handle of a wooden spoon is put in the oil it sizzles. Working in batches of 4 (or how many your pan with hold), fry the samosa until the bottom is golden brown, about 4-5 minutes, turn and repeat. Drain on a paper towel.