In a medium pot, add rinsed lentils and cover with water. Bring to a boil, reduce heat, and simmer until tender (about 25-30 minutes). Drain and set aside.
In a large saucepan, heat vegetable oil over medium heat. Add chopped onions and sauté until golden brown.
Stir in the flour and cook for 1-2 minutes to create a roux.
Gradually whisk in vegetable broth until smooth. Bring to a gentle simmer.
Add cooked lentils, vinegar, sugar, salt, pepper, and marjoram if using. Let it simmer for 10 minutes, stirring occasionally.
Meanwhile, cook Spätzle noodles according to package instructions or prepare homemade Spätzle.
Serve the lentils warm over Spätzle and garnish with fresh parsley.