In a large saute pan, on medium heat, add a small amount of water and the onions. Cook until the onions become translucent. Then add the garlic and mushrooms and continue to cook for a couple more minutes until the mushrooms start to soften.
Lastly, add in the lentils, sage, salt and water. Cover and let cook about 15 minutes or until the green lentils soften. I like my lentils with a little bite, so I didn't cook them as long.
This should last at least a week in the fridge. You can also freeze it for several months. Enjoy!