Trim ends off of green beans. Bring a pot of water to boil. Add salt. Working in batches, blanch green beans just until they turn bright green but are not fully tender, then drop them in a bowl of ice water to stop the cooking process. Remove from the water and let air dry. Continue until you’ve done this to all your green beans.
Heat 1 tablespoon of olive oil in a large skillet over medium high heat. Cook the mushrooms for several minutes until they begin to brown on one side. Flip the mushrooms and let them get more color on the other side. Add 2 tablespoons of butter and thyme sprigs to pan. Continue to cook until the butter begins to brown and mushrooms are tender. Season with salt and pepper. Repeat process with the other half of the mushrooms. Remove thyme sprigs before adding to green bean mixture.
Melt remaining 2 tablespoons of butter in a large sauce pan over medium low heat. Whisk in flour and cook, whisking constantly until the roux is golden brown and smells mildly nutty. Whisk in milk and cream and increase the heat bringing the sauce to a simmer. Whisk regularly until it thickens. Remove from heat and add Seahive and garlic, whisking as you add these ingredients. Season with salt and pepper.
Mix together the green beans and mushrooms, then place in a 2-quart baking dish. Pour sauce over the green beans. Cover tightly with foil and bake until sauce is bubbling, about 25–30 minutes. Uncover and continue to bake until casserole is lightly browned on top and around the edges, about 15–20 minutes longer. Top with fried onions and continue to bake just until onions are slightly darker and fragrant, about 3 minutes. Let sit 10 minutes to set before serving.