Prepare the topping: Pour vegetable oil in a Lodge Logic 7 quart dutch oven, about 2” deep. Heat to 350ºF on a side burner. Toss the shallots in the flour and Cattleman’s Grill California Tri-tip Seasoning. Fry the shallots in a couple of batches, for about 1 minute, until crispy and golden. Use a slotted spoon to remove the shallots and place on paper towels; season with salt, to taste. Combine the fried shallots with the panko and 1/4 cup parmesan in a bowl. Toss to combine. Set aside. Discard the oil.
Turn on your side burner to high heat. Cook green beans in boiling salted water in the dutch oven for 3-5 minutes, to desired degree of doneness; drain. Place in cold water to stop the cooking process.
Return the empty dutch oven to the side burner. Over medium heat, melt the butter, then add the mushrooms, onion and Cattleman’s Grill California Tri-tip Seasoning. Cook until the onions are translucent and mushrooms are softened, about 10 minutes. Stir in the flour and garlic and cook 1 minute, stirring constantly. Stir in the wine and continue to cook and stir for 1 minute. Stir in the half and half and cook and stir until sauce is thickened and bubbly. Stir in the parmesan. Remove from heat.
Fold the green beans into onion and mushroom sauce. Taste and adjust seasoning, as needed. Sprinkle the fried shallot topping mixture over the green beans and sauce.
Preheat your gas grill to 400ºF, set up for indirect grilling. Bake the green bean casserole until it’s bubbling and the topping is crispy, about 45 minutes.