Finely chop the onion and garlic and set aside. In a medium bowl, combine the red lentils, bulgur, and rice. Rinse them well and drain. Set aside.

In a large pot over medium heat, add the butter and olive oil. Stir until the butter melts completely. Add the chopped onion and sauté for about 2 minutes, until softened.

Add the garlic and sauté for 30 seconds. Then add the tomato paste and stir. Cook for 1 minute, stirring to prevent sticking.

Add the spices: salt, red pepper flakes, cumin, and black pepper. Stir to combine. Add the lentil mixture and stir well to coat with the tomato mixture.

Pour in the 6 cups of water and increase the heat to bring to a boil. Once boiling, reduce the heat to medium and simmer for 20 to 25 minutes, or until the lentils, bulgur, and rice are cooked and the soup is thick. Stir occasionally to prevent sticking.

Taste and add more salt if needed. Once everything is cooked, remove the pot from the heat and stir in the lemon juice.

Ladle the soup into serving bowls and garnish with a sprinkle of dried mint and a little red pepper flakes, if desired. Serve hot.

Notes:
- The soup should be thick and hearty, but if it becomes too thick, you can add more water to reach your desired consistency.
- Let the soup cool completely, then store it in an airtight container in the refrigerator for 3 to 4 days.
- You can also freeze the soup for later use. It will keep in the freezer for 2 to 3 months. To use after freezing, thaw it in the refrigerator overnight, then reheat in a pot on the stove. Add more water if it is too thick to reach your preferred consistency.