Put ground beef into a bowl and sprinkle with baking soda. Mix well. Set aside.
Add butter and olive oil to the soup pot. Once it begins to sizzle, add ground beef and cook until nicely browned. Approximately 5-7 minutes.
Add salt, pepper, red pepper flakes (if using), and Italian seasoning. Mix well with ground beef.
Add onions and sauté until translucent. Approximately 1-2 minutes.
Add garlic and sauté until fragrant. Approximately 1-2 minutes.
Add potatoes and sauté until the potatoes absorb some of the fat at the bottom of the soup pot and begin to release their starch. Approximately 2 minutes.
Add liquid and bay leaves. Bring to a medium simmer, and allow the potatoes to cook until almost tender. Approximately 10 minutes. (Cooking time will depend on the size of the potatoes.)
Add tomatoes and tomato paste and stir well to incorporate.
Add spinach and mixed vegetables and stir well to incorporate.
Simmer soup until the tomatoes, spinach, and vegetables are warmed through.
Serve soup topped with optional Hamburger Bun Croutons. (See video and RECIPE NOTES below.)
Soup can be stored well wrapped in the refrigerator for 3-4 days and in the freezer for 2-3 months. To reheat, warm gently in a soup pot on the stovetop until warmed through.