While heating the pasta water, throw a can of crushed (or anything else, really) tomatoes in another pot and heat. Add some vegetable stock and some spices. When the pasta water boils, throw in red lentils into the tomato sauce and both parts will be done at the same time. The sauce will be enough for several servings. With decent spices, it can be very nice.

Bonus points for starting off with browning some onions before adding the tomatoes.