Bring 2 C of water to boil. Place porcini in heat proof bowl and pour the boiling water over the porcini. Cover for 30 minutes to soften the porcini.
Put the black rice on to cook. Fill stock pot with rice, a pinch of sea salt and 2 C water. Bring to a boil, turn the stove to low, cover and cook for about 30 minutes. When cooking is complete, the rice will be tender and no water will remain. Place rice in large mixing bowl. Set aside.
Cut the onions in half and slice length wise so that your slices are thin and long. Melt the coconut oil and 1 Tbs butter in a large non stick sauté pan. Place the onions in the pan with 1/2 tsp salt and 1/2 tsp pepper. Cooking on medium to low heat, these will take about 30 minutes to caramelize. Don't rush them else they'll burn. Low and slow here. Stir occasionally.
For the greens, wash the kale and chard. De-stem the kale. Ribbon the greens by rolling the leaves up, length wise, then cutting about 1/2" ribbons. Then, rough chop the ribbons in 1/2. Set aside.
Melt 1 Tbs butter and mix it into the bread crumbs. Set aside.
Preheat the oven to 375F (190C). After the porcini have softened, strain the porcini through cheese cloth, reserving the broth. Squeeze the cheese cloth so that the broth is extracted from the porcini. Give the mushrooms a good rinse in a sieve to make sure they are free from debris. Rough chop the porcini and place them in the large mixing bowl with the rice. Strain the broth through the sieve again. Measure out 1/3 C (80g) of the broth and set aside. Freeze or refrigerate the remaining broth for future use.
When the onions are done caramelizing, add the greens to the sauté' pan, with the onions. This may be done in batches if needed. When the greens just start to wilt, pull them from the pan and put them in the large mixing bowl with the cooked rice and porcini.
In the large mixing bowl, mix all the rice, porcini, greens, caramelized onions, 1/2 the grated gruyere, thyme, nutmeg, and 1/2 tsp salt and 1/2 tsp pepper. Mix to evenly distribute the ingredients. Taste to check seasoning. Now is a good time to adjust if necessary.
Prepare a 3-4 Quart casserole by spraying it with cooking spray. Pour the rice/greens mixture in the dish, smoothing it out. Pour a 1/3 C (80g) of the broth over the mixture. Spread the remaining gruyere over the top, then top the gratin with the breadcrumbs. Bake uncovered for 25 minutes. Garnish with fresh, chopped thyme.
Store in a covered container for up to three days.