Preheat the oven to 375 degrees.
Bring a large pot of salted water to boil.
In a large cast iron pan or casserole dish, melt 1 tablespoon of ghee or butter over medium-low heat. Add the shallots and cook until translucent, stirring occasionally, about 2 minutes. Add the mushrooms, the white parts of the scallions, and the tamari, and continue to cook until the mushrooms are tender, about 5 more minutes. Deglaze the pan with the wine and cook until all the liquid has nearly been absorbed by the vegetables, about 3 minutes. Turn the heat off and scrape the mushroom mixture into a bowl. Set aside.
Add the remaining ghee or butter to the pan and melt it over low heat. Whisk in the flour then pour in the coconut milk. Whisk or stir with a wooden spoon until there are no lumps. Turn the heat back up to medium and simmer the sauce until is thick enough to coat the back of a spoon, stirring continuously, about 5 minutes. Remove from the heat.
In the meantime, cook the pasta according to package directions until just shy of al dente (about 8 minutes, versus the 12 my box called for). Drain in a colander.
Off the heat, add the mushroom mixture, green parts of the scallions, tuna, lemon juice, salt and red pepper flakes to the béchamel. Stir until combined. Gently fold in the pasta until fully distributed. Top with breadcrumbs and bake in the oven for 15 minutes, until bubbling and browned. Serve immediately.