In a small bowl, combine salsa, tomato paste, olive oil, garlic, lime juice, pepper and salt.
Lay the flank steak in the slow cooker and pour the mixture over it, making sure to coat all exposed surfaces well. Lay the onion and bell peppers on top.
Close and lock the lid of the multi-cooker and set the regulator knob to the STEAM position. Set the timer for 5½ hours on slow cook LOW.
When time is up, remove the steak and vegetables from the juice and transfer them to a serving platter. Cover with aluminum foil and let stand 10 minutes.
Cut the meat across the grain into ½-inch thick slices. Serve it heaped over warm tortillas, with the peppers and onions on top. Garnish with chopped tomatoes if desired.