Pre-heat the oven to 375 degrees. Rub a medium baking dish with a little olive oil and set aside. (You could also use a cast iron skillet if you like!)
Combine the brown rice and mushroom stock in a medium pot and bring to a boil. Lower to a simmer, cover, and cook for 45 minutes until the liquid has been absorbed. Remove from the heat and let rest for 5-10 minutes, covered, then fluff with a fork.
Place the strips of bacon on a rack over a foil lined baking sheet. Bake for 25 minutes until brown and crispy. When cool enough to handle, crumble the bacon and set aside until ready to use.
While your bacon is in the oven, prepare all of the other ingredients for the casserole. Slice the leeks and mushrooms, mince the onions and garlic, chop the herbs, rinse the beans, and grate the cheeses. (Trust me, having everything ready to go by the stove will make your life infinitely easier, friends.)
When you have all your ingredients ready to go, you’re ready to start cooking. Heat the olive oil in a large sauté pan over medium heat. When hot, add the leeks, onions and garlic and cook until soft, about 5 minutes.
Add the mushrooms, thyme, and sage and cook for 8 minutes until the mushrooms have released their liquid and are tender.
Add the cannellini beans and cook for 1 minute until heated through. Stir in the cooked brown rice and turn off the heat. Add 2/3 of the crumbled bacon and 2/3 of the fontina and gruyere and mix until well combined. Taste and season with a healthy amount of fresh ground pepper and a little kosher salt if needed.
Turn out the rice mixture into the prepared baking dish and top with the remaining fontina and gruyère.
In a small bowl, combine the Panko and Parmesan cheese. Sprinkle the mixture in an even layer over the casserole.
Transfer the casserole to the oven and bake for 25 minutes until the cheese is bubbling and the top is lightly browned. Garnish with the remaining crumbled bacon and chopped parsley and serve hot.