How to Make It
Prepare the Stew: Preheat oven to 350°F. Heat a large Dutch oven over medium-high. Add ground chuck, mustard, oregano, and 1 teaspoon of the salt; cook, stirring occasionally, until crumbled and browned, about 8 minutes. Using a slotted spoon, transfer beef mixture to a bowl lined with paper towels, reserving 1 tablespoon drippings in Dutch oven.
Return Dutch oven to medium heat, and add onion. Cook, stirring often, until softened, about 4 minutes. Add garlic and tomato paste; cook, stirring constantly, until fragrant and warmed through, about 1 minute. Add flour; cook, stirring often, 1 minute. Add broth, tomatoes, and ketchup, scraping bottom of pan to loosen browned bits. Stir in cooked beef. Increase heat to medium-high, and bring to a boil. Add potatoes and remaining 1/2 teaspoon salt; reduce heat to medium. Simmer until potatoes are tender, about 20 minutes. Stir in mixed vegetables, and cook until warmed through, 2 to 3 minutes.
Prepare the Croutons: While potatoes cook, toss together bun pieces, olive oil, and salt on a parchment paper-lined baking sheet; spread into a single layer. Bake in preheated oven until golden and crisp, about 10 minutes. Serve with Stew.