Sauté the meatball aromatics: Heat 1 tbsp olive oil in a large skillet over medium-high heat. Add the finely diced onion and sliced garlic from the meatball list with 1/2 tsp salt, cook 4 to 5 minutes until soft and sweet, and stir in the red-pepper flakes for the last minute. Tip into a large bowl and let cool for 10 minutes — warm onions start cooking the meat and ruin the texture.
Soak the breadcrumbs: While the onions cool, put the breadcrumbs in a small bowl and pour over the milk. Let them sit 2 to 3 minutes until fully absorbed and mushy — this milk-soak, the panade, is what keeps every meatball soft instead of dense.
Mix and roll the meatballs: Add the ground beef to the bowl with the cooled onion mixture and the soaked breadcrumbs. Season generously with 1 1/2 tsp kosher salt and 1/2 tsp black pepper. Mix with your hands just until combined — overworking makes dense meatballs. With wet hands, roll into balls just smaller than a golf ball (about 1 1/4 inches; makes roughly 24). Arrange on a parchment-lined tray and chill for 30 minutes until firm.
Brown with the cumin: Heat 2 tbsp olive oil in a large heavy-bottomed pot (a Dutch oven is ideal) over medium-high heat. Add the diced onion and sliced garlic for the soup and cook 4 to 5 minutes until softened. Add the cumin seeds and the meatballs together, then cook over high heat, turning the meatballs, for 6 to 8 minutes — the cumin toasts in the rendered beef fat and the meatballs color on all sides.
Build the broth: Stir in the minced chipotles in adobo and cook 1 minute until fragrant. Pour in the chopped tomatoes, oregano and stock. Bring to a boil, then lower the heat, season with salt and pepper, and simmer gently for 20 minutes until the meatballs are cooked through and the soup has thickened slightly.
Add the vegetables: Stir in the sweetcorn and diced zucchini and cook 3 to 4 minutes until just tender. They need no longer — any earlier and they go mushy and flavorless.
Serve: Ladle into warm bowls. Top with the crushed tortilla chips, chopped cilantro and jalapeño peppers, and serve immediately while the chips are still crunchy.