Preheat oven to 375°F. Line a baking sheet with parchment paper. Cook lentils according to package directions.
In a large bowl, mix garlic, onion, celery, leeks, flaxseed, Dijon, tomato paste and spices. When lentils are cooked, mash them with the back of a fork, leaving some whole. Add mashed lentils and rice to bowl. Mix all ingredients together.
Heat oil in a medium saucepan on medium-high. While oil is heating, form lentil-rice mixture into 6 lentil patties. Add them to pan and cook for 2 to 3 minutes per side, until light brown. Transfer patties to prepared baking sheet and cook in oven for 15 minutes to warm through. Serve with a side salad and topped with diced tomatoes and avocado, if desired.