Preheat oven to 425F (218C) and place oven rack in the center of the oven. Spray a 9 1/2" (24cm) gratin or casserole dish with cooking spray. Set aside. Line a sheet pan with parchment paper. Using your hands or a brush, spread one Tbs of olive oil all over the parchment paper. Set aside.
Cut the fennel fronds off the bulb and chop the herbaceous parts to yield about 1 Tbs. Set aside. Trim the top of the fennel and just a little off the base. You'll want to keep the fennel in tact at the base as much as possible so that the wedges won't fall apart. Cut fennel in half from top to bottom, then slice in 1/4" (25mm) wedges placing the wedges close together but not touching on the sheet pan. Brush tops, throughly with remaining olive oil (you may have some olive oil left over and this will vary depending on the size of the fennel). Roast in oven for 30 minutes, stopping to check on them at 25 minutes. They should caramelize (turn a dark color but not burn). Some will caramelize more so than others. This is okay.
While the fennel is roasting, cook the brown rice according to package directions. Add the cooked brown rice to a medium mixing bowl (it's okay if it cools off).
In a large sauté pan add 2 tsp of olive oil and heat until shimmering. Add the mushrooms, spreading them out in a single layer as best you can and stir only occassionally, about every two minutes for 9-10 minutes. We want them to sear. Add the onion in with the mushrooms and sauté for about 3 minutes or until the onions are just soft. Add the mushroom/onion mixture to the mixing bowl. Mix in the beans, Gruyere, zest of 1/2 a lemon, 2 tsp of the minced fennel fronds, juice of one lemon, broth, 1/2 tsp salt and 1 tsp ground pepper. Taste to adjust seasoning as needed. Spoon into prepared gratin or casserole dish.
To make the crust, in a small bowl, mix the breadcrumbs, dijon, parmesan, pinch of salt and zest of 1/2 a lemon. Set aside.
Reduce oven temperature to 400F (204C). Arrange fennel wedges over the top of the rice mixture, flipping each wedge over so that the darkest caramelized side is up, shingling if needed to fit all the fennel on top. Spread the bread crumbs evenly over the gratin. Bake uncovered for 20 minutes.
Sprinkle remaining cut fennel fronds over the top. Serve with lemon wedges. Store in covered container for up to three days. When reheating, add a few tsp of water and cover. Reheat at 350F (180F) for about 20 minutes.