In a skillet over medium heat, add the olive oil and butter. Once melted, add the sweet and yellow onions, stirring occasionally. Cook until the onions are caramelized, about 15-20 minutes.
Once the onions are golden and fragrant, add the minced garlic, thyme, sea salt, and ground black pepper. Sauté for an additional 2-3 minutes until the garlic is fragrant.
Transfer the caramelized onion mixture to the slow cooker. Pour in the beef broth and give it a gentle stir to combine all the flavors.
Cover the slow cooker and set it on low for 6-8 hours or high for 3-4 hours.
About 30 minutes before serving, preheat your oven to 375°F (190°C). Place the whole-wheat bread slices on a baking sheet and toast them until golden brown, about 10 minutes.
Once the soup is finished cooking, ladle it into bowls. Top each bowl with a slice of toasted bread and a slice of provolone cheese. Place under the broiler for 2-3 minutes until the cheese is bubbly and golden.
