Preheat oven to 350.
Pull the mushroom caps off the stems and trim both parts. Roughly chop the stems and measure – you want a total of 2½ cups, so make up for any shortfall with roughly chopped caps. Save the rest of the caps for another recipe.
Transfer chopped mushrooms to food processor and add breadcrumbs, lentils, flax eggs, garlic, cornstarch, mustard, and herbs. Process into a smooth paste, the consistency of split pea soup.
Heat 2 t oil over medium heat, add the onion, and sauté until translucent, about 5 minutes. Add tomatoes and cook another 2 minutes. Add this to the food processor and pulse a few times to combine with the mushroom mixture.
Transfer to a bowl and stir in the rice, salt, and pepper.
Oil the cake pan or ramekins and spread the mixture in an even layer. Half fill the large baking dish with hot water, then carefully set the smaller dish(es) inside.
Bake uncovered for 45 – 60 minutes, until dark, crusty, and firm in the middle.
Let cool for 10 minutes, then run the knife along the edges to loosen the meatloaf. Place a plate on top and hold it there while you turn the baking dish upside down.
Serve hot topped with mushroom-miso gravy and a side of mashed potatoes.