Prep all ingredients: Peel and cut carrots, shred cheese, and measure out all components for the sub and chia fresca.
Make the sauce: Place the marinara sauce in a pot over medium heat. Cover with a lid and bring to a low simmer. If using homemade sauce, see the Marinara Sauce recipe below.
Make the meatballs: In a bowl, combine the chopped bread and milk. Let sit for 2–3 minutes. Add ground beef, oregano, parmesan, salt, and pepper. Mix until well combined. Using your hands, form the mixture into equally-sized meatballs.
Heat a sauté pan over medium heat. Add 1 TBSP olive oil and sear the meatballs in batches, without crowding the pan, until browned on all sides. Transfer meatballs to the simmering sauce. Cook until the internal temperature reaches 160°F (71°C). Taste and adjust sauce seasoning with salt and pepper. Remove from heat.
Preheat the oven broiler on low.
Prep the sub rolls: Drizzle the inside with olive oil. Place on a baking sheet and lightly toast under the broiler.
Assemble the subs: Place meatballs on each roll, spoon over some sauce, and top with shredded mozzarella. Place under the broiler until the cheese is melted and bubbly.
Make the sparkling chia fresca: combine all the ingredients together. Stir well and let sit for 5 minutes to allow the chia seeds to bloom.
Serve the meatball subs with carrots and ranch, and chia fresca.