Add 6 cups of water to a medium or large pot along with the two cups of red or yellow lentils. You can always add an extra 1/2 cup if the soup is thicker than you'd like.
Bring the water and lentils up to a boil and then reduce to a low simmer.
After about 20 to 30 minutes and the lentils are very soft, turn off the heat and cover the pot.
Chop a large sweet onion and set it aside.
In a large pan add a tablespoon of vegetable oil and set the stove to medium heat to get the pan warm.
Add the onions to the pan with the vegetable oil in it once the oil is warm and saute the onions until they are translucent.
Add a tablespoon of crushed cumin to the onions and mix all together. You can add more or less cumin depending on how you like it. If you are crushing your own cumin from whole seeds, then use 1/2 a tablespoon to start just in case the spices are strong.
Once well mixed and heated throughout, add the onion and cumin mixture to the pot of lentils.
Add two teaspoons of kosher salt to the lentils. Add more salt to taste.
Use a hand blender to puree the lentils and onions until smooth.
Serve the lentil soup plain, with a side of crusty bread, over rice, or over toasted pita bread.