Green Bean Casserole with Almost-Burnt Almonds

Preheat oven to 350°F. Spread almonds in an even layer on a rimmed baking sheet. Bake in preheated oven until well browned inside and out, 12 to 15 minutes. Let cool slightly, about 5 minutes. Chop and set aside. (Do not turn oven off.)

Bring a large pot of salted water to a boil over high. Add green beans; cook, stirring occasionally, until crisp-tender, about 4 minutes. Remove from heat. Using a slotted spoon, transfer beans to a bowl filled with ice water; let stand 2 minutes. Drain well, and pat dry; set aside. Discard water in pot; wipe pot clean.

Add butter and 2 tablespoons oil to pot; cook over medium until butter melts. Add onion, garlic, and 1 1/2 teaspoons salt; cook, stirring occasionally, until softened and well browned, 8 to 10 minutes. Stir in stock base and crushed red pepper; cook, stirring occasionally, 30 seconds. Add flour; cook, stirring constantly, until mixture is bubbling, about 2 minutes. Gradually whisk in milk and 2 cups water; bring to a boil over medium-high. Reduce heat to medium-low; simmer, whisking constantly, until mixture thickens, about 5 minutes. Remove from heat; whisk in vinegar. Fold in green beans, black pepper, and remaining 1 1/2 teaspoons salt.

Grease a 3-quart baking dish with butter; spoon green bean mixture evenly into dish. Stir together chopped almonds, panko, and remaining 1 tablespoon oil in a bowl; sprinkle evenly over green bean mixture. Bake at 350°F until casserole is bubbling and almonds are deeply toasted, 20 to 25 minutes. Remove from oven; let stand 10 minutes.