Start cooking the green lentils according to directions on the package and add 1 bay leaf. When tender, strain, return to pot and then add salt to taste.
In a large heavy bottomed skillet over medium high heat, melt the butter until it's foamy, then saute the onion for 2 minutes. Add the finely grated carrots and cook 5 minutes more.
Reduce the heat to medium, then make a hole in the center and melt more butter before adding the mushrooms. Cook for 8 minutes until soft.
Stir in the cooked green lentils with a large wooden spoon, then add the dry spices (ground coriander, mixed spice, nutmeg, black pepper and salt) and the flavorings (brown sauce, Marmite, and molasses/black treacle).
Pour in the vegetable stock and add the steel cut oats (US) or pinhead oats (UK). Stir until well mixed then reduce the heat to medium low and simmer for 15 minutes. Make sure you stir it every couple of minutes so the mixture doesn't stick to the bottom of the pan. Taste and adjust the seasonings, adding more freshly cracked black pepper, salt, or butter to get it just right.
While the vegetarian haggis is simmering on the stovetop, prep a 1.5 L pudding basin by brushing the inside with butter and placing a circle of parchment paper in the bottom.