Cook the beef: Place the eye of round in the Instant Pot with water to cover, salt, halved onion, smashed garlic, vegetables, bay leaf, peppercorns, coriander seeds, and bouillon. Seal and pressure cook on high for 20 minutes. Allow natural pressure release for 15 minutes, then release remaining pressure manually.
Cool and slice: Remove the beef and set it on a cutting board to cool completely, about 30 to 45 minutes. Reserve ½ cup of the cooking broth before discarding the rest. Once fully cool, slice the beef into thin rounds. The thinner the slice, the better the vinagreta absorption overnight.
Make the vinagreta: Whisk together the vinegar, olive oil, reserved broth, and mustard until combined and emulsified. Stir in the minced garlic, diced onion, red pepper, leek, and sliced olives. Taste and adjust salt and vinegar to your preference.
Assemble and refrigerate: Arrange the beef slices on a serving platter, overlapping slightly. Pour the vinagreta generously over the top, making sure every slice is covered. Spoon any remaining vegetables and olives over the beef. Cover tightly and refrigerate for at least 4 hours. Overnight is better.
Serve: Remove from the fridge 10 minutes before serving. Taste the vinagreta that has pooled on the platter and adjust if needed. Serve cold with crusty bread, tostones, or white rice alongside.