Preheat oven to 375°F (190°C). Line 9 x 5-in. (23 x 13-cm) loaf pan with parchment paper. Heat oil in skillet set over medium heat. Cook onions, garlic and rosemary 3 to 5 min., until softened; set aside. Puree half the lentils in food processor with broth until coarsely ground.
In Large bowl, mix together ground lentils, remaining lentils, onion mixture, oats, ground flax, tamari, tomato paste, mustard and eggs. Press into prepared loaf pan. Bake 45 to 50 min. until golden brown and loaf starts to shrink from the sides.
Meanwhile, make the gravy. Stir together onions, garlic, rosemary and oil in small baking dish (about 8-in./20-cm square). Bake in same oven as loaf, stirring occasionally, 45 to 50 min. until onions are golden brown and tender. Place onion mixture, broth, mustard and tamari in blender and puree until smooth. Pour into small saucepan and bring to a boil. Reduce heat to medium-low. Simmer 5 min. until thick and flavourful.
After loaf is baked, cool in pan 10 to 15 min. before turning out onto serving dish. Slice and serve with gravy, sprinkled with green onions.
Brush loaf with ketchup before baking for a kid-friendly version. Serve with additional ketchup on the side, saving the onion gravy for the adults.