Saute the sliced onions with the salt and the pepper in the olive oil and butter on medium heat until well caramelized. Be patient with this process. The onions should be light brown in colour.
Add the minced garlic and dried chili flakes. Continue to sauté for another 2 minutes.
Add the drained and rinsed lentils and cook until heated through - about 2-3 minutes should do it.
Lastly, stir in the mustard and the sour cream and simmer for 3 minutes.