- Preheat oven to 350 °F.
- Combine water, Lentils, and ¼ teaspoon salt in a large saucepan. Bring to a boil; cover, reduce heat to medium-low, and simmer 20 minutes.
- Drain Lentils in a colander over a bowl, reserving 1 cup cooking liquid.
- Combine Lentils and diced onion in an 11x7-inch baking dish.
- Combine ¼ teaspoon salt, reserved cooking liquid, ketchup, and the remaining ingredients.
- Pour the ketchup mixture over the Lentil mixture, stirring to combine.
- Bake at 350 °F for 1 hour.