Heat a large, heavy-bottomed pot or Dutch oven over medium-high heat. Add olive oil and season the bone-in beef short ribs with salt, black pepper, garlic powder, and thyme. Sear ribs for about 4-5 minutes on each side until browned. Remove and set aside.

Melt butter in the same pot over medium heat. Add thinly sliced yellow onions, stirring to coat, and sprinkle with sugar. Cook onions for 25-30 minutes, stirring frequently, until golden brown and caramelized.

Stir in minced garlic and cook for another minute. Sprinkle all-purpose flour over the onions, mixing well. Cook for an additional minute to slightly brown the flour.

Return seared short ribs to the pot and add beef broth, Worcestershire sauce, and bay leaf. Stir to combine, ensuring ribs are submerged. Bring to a gentle simmer.

Cover pot with a lid and reduce heat to low. Slow cook for 3 hours on stovetop or transfer to a slow cooker for 7-8 hours until ribs are fork-tender.

After slow cooking, adjust seasoning with salt and pepper. Remove bay leaf and preheat oven to 400°F (200°C). Prepare Gruyère toast by slicing baguette and placing slices on a baking sheet.

Brush baguette slices with melted butter and top with grated Gruyère cheese. Bake for 8-10 minutes or until cheese is bubbly and golden brown.

Ladle hot soup into bowls and serve immediately with a slice of cheesy toast on top.