Preheat oven to 275 degrees F (135 degrees C). Spread almonds on a baking sheet.

Toast almonds in the preheated oven until golden, stirring halfway through, about 13 minutes total. Remove from oven and set aside.

Increase oven temperature to 350 degrees F (175 degrees C). Grease one 9x13-inch casserole dish.

Place bacon and onion in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, 8 to 10 minutes. Remove bacon and onion with a slotted spoon; set aside. Drain excess bacon fat from the skillet; add olive oil. Add mushrooms and cook, stirring, until soft, about 5 minutes.

Place 1 quart water in a saucepan and bring to a boil. Add green beans and blanch for 2 minutes. Drain in a colander and immediately immerse in ice water for several minutes to stop the cooking process and preserve the bright green color. Drain.

Pour mushroom soup into a large bowl and whisk until smooth, adding up to 1/2 cup water, until soup is the consistency of thick gravy. Stir in almonds, 3/4 of the bacon-onion mixture (reserving the rest for topping), mushrooms, green beans, 1 cup French-fried onions, and water chestnuts. Mix thoroughly to combine and season with salt and black pepper.

Pour green bean mixture into casserole dish. Top with remaining bacon-onion mixture and 1 cup French-fried onions.

Bake in the preheated oven until bubbling and crispy on top, 30 to 45 minutes. Serve hot.