Prepare the grill for direct cooking over high heat (450° to 550°F). If you're using a charcoal grill, keep a small part of the charcoal grate clear of briquettes as a safety zone of indirect heat.
Brush the cooking grates clean. Grill the bell pepper over direct high heat, with the lid closed, until blackened and blistered all over, 10 to 12 minutes, turning occasionally. Place the pepper in a small bowl and cover with plastic wrap to trap the steam. Set aside for at least 10 minutes, then peel the skin from the pepper, discard the stem and seeds, and roughly chop the pepper.
Brush the cooking grates clean. Grill the bell pepper over high heat, with the lid closed, until blackened and blistered all over, 10 to 12 minutes, turning occasionally. Place the pepper in a small bowl and cover with plastic wrap to trap the steam. Set aside for at least 10 minutes, then peel the skin from the pepper, discard the stem and seeds, and roughly chop the pepper.
In a small skillet over medium heat on the stove, toast the coriander, mustard, and cumin seeds until the aromas of the spices are apparent, 3 to 5 minutes, stirring occasionally. Pulverize the spices in a spice mill. In a food processor finely chop the garlic. Add the chopped bell peppers, pulverized spices, and the remaining sauce ingredients, including ½ teaspoon of the salt. Process until completely smooth.
Allow the steaks to stand at room temperature for 15 to 30 minutes before grilling. Lightly brush both sides of the steaks with oil and season evenly with the remaining ¾ teaspoon salt and the pepper. Grill over direct high heat, with the lid closed, until cooked to your desired doneness, 6 to 8 minutes for medium rare, turning once or twice (if flare-ups occur, move the steaks temporarily over indirect heat). Remove from the grill and let rest for 3 to 5 minutes. Serve warm with the sauce on the side.
Allow the steaks to stand at room temperature for 15 to 30 minutes before grilling. Lightly brush both sides of the steaks with oil and season evenly with the remaining ¾ teaspoon salt and the pepper. Grill over high heat, with the lid closed, until cooked to your desired doneness, 6 to 8 minutes for medium rare, turning once or twice (if flare-ups occur, move the steaks temporarily over indirect heat). Remove from the grill and let rest for 3 to 5 minutes. Serve warm with the sauce on the side.
