Heat the second Tablespoon of oil in a deep cast iron skillet over medium-high heat with a bit of mushroom. When it begins to sizzle, add the remaining mushrooms and cook, stirring often, until they release their liquid and begin to turn golden brown.
Stir in the cayenne, black pepper, garlic, salt, thyme and continue to cook for about 30 seconds.
Reduce the heat and slowly stir in the vegetable broth and coconut milk. Scrape the bottom of the pan with a wooden spoon to release any bits of mushrooms. Simmer, partially covered, for about 3 minutes; stir occasionally.
Spoon about 1/4 cup of the gravy into a small bowl and let it cool down for a couple of minutes. Add the arrowroot powder and whisk until smooth, then slowly pour back into the skillet while stirring. Simmer, stirring, until thickened (this will happen almost immediately)
Cut or snap the beans in half and arrange in the gravy so they are submerged. Simmer partially covered until the beans are just tender. Baby french beans cook in only 4 minutes; larger beans will take longer. Don’t overcook.
Sprinkle the bread crumbs (or cracker crumbs) on top of the casserole, next sprinkle the paprika, and then arrange the sautéed onions.
Place under the broiler (not too close) for about 20 seconds until some of the onions are a bit crispy.