Heat 1 tbsp (15ml) of olive oil in a large skillet over medium-high heat. Add 1 cup (150g) of diced yellow onion and cook until softened. Add 1 lb (450g) ground beef and sear until a deep golden-brown crust forms. Stir in 3 cloves (15g) minced garlic for the last minute, then drain excess grease.

In a bowl, whisk together 1.5 cups (360ml) beef broth, 2 tbsp (15g) cornstarch, 1 tbsp (15ml) Worcestershire sauce, 1 tsp (6g) salt, and 1/2 tsp (1g) black pepper until perfectly smooth.

Transfer the browned beef mixture to the black slow cooker insert. Top with 4 cups (600g) diced russet potatoes, the drained red kidney beans, and 1 cup (150g) sweet yellow corn. Pour the gravy mixture evenly over the top and stir gently to combine.

Cover tightly and cook on LOW for 6 to 8 hours (or HIGH for 3 to 4 hours) until the diced russet potatoes are perfectly tender when pierced with a fork.

During the last 15 minutes of cooking, remove the lid and evenly sprinkle 1 cup (115g) cheddar cheese and 1 cup (115g) Monterey Jack cheese over the casserole. Cover and cook until the cheese is glossy and bubbling.

Turn off the heat. Sprinkle the bubbling cheese layer with 1/4 cup (25g) fresh chopped green onions. Serve hot directly from the crockpot.