Wipe or rinse mushrooms to remove any dirt. Trim stem ends (entire stem for shiitakes), slice about 1/4 inch thick and set aside.
Heat coconut oil over medium-low heat in a large skillet. Add shallot and garlic and cook, stirring, until they start to soften and turn slightly golden, about 4-5 minutes. Add mushrooms, and continue to cook and stir until tender, about 10-12 minutes. Stir in tamari and pepper and cook and stir until liquid cooks off, a few minutes more.