Season short ribs generously with salt and pepper; place in a large roasting pan and chill, uncovered, 12 hours.
Preheat oven to 325°F. Toast breadcrumbs on a rimmed baking sheet, tossing halfway through, until golden brown, 8–10 minutes. Let cool.
Add 1 cup water to roasting pan. Cover pan with foil and cook until meat is tender, 2 1/2–3 hours. Uncover pan and increase oven temperature to 400°F. Roast until ribs are browned on top, 25–30 minutes longer.
When short ribs are almost done, mix garlic, parsley, horseradish, lemon zest and juice, breadcrumbs, and 2 tablespoons oil in a small bowl; season gremolata with salt and pepper.
Meanwhile, heat remaining 1 tablespoon oil in a large skillet over medium-high heat. Cook lemons, cut side down, until golden brown, about 4 minutes. Top short ribs with gremolata and serve with seared lemons.