Trim any excess fat from the beef chuck roast and cut it into 1-inch cubes.
Place the beef cubes into the slow cooker and season with the sea salt, black pepper, and dried oregano.
Mince the garlic and add it to the slow cooker along with the tomato puree, stirring to coat the beef.
Cover and cook on low for 7-8 hours or on high for 4 hours until the beef is fork-tender.
When the beef is nearly finished, preheat your oven to 400°F and line a baking sheet with parchment paper.
Slice the zucchini into rounds, chop the red bell pepper into bite-sized pieces, and cut the red onion into thick wedges.
Toss the vegetables with the olive oil on the prepared baking sheet and roast for 20-25 minutes until tender and caramelized.
Plate the slow-cooked beef with a spoonful of its cooking sauce and serve the roasted vegetables on the side.
Garnish the dish with crumbled feta cheese, kalamata olives, and freshly chopped parsley.