Preheat oven to 400°F (200°C). Slice onion into 1–inch (2.5 cm) wedges. Place on a baking sheet and toss with 1 tsp (5 mL) oil. Roast, stirring occasionally, 10 minutes.
Meanwhile, if tomatoes are large, slice in half. Discard seeds from jalapeno if you don't want the relish to be super hot. Add both to roasting pan along with remaining 2 tsp (10 mL) oil and salt. Toss. Continue to roast just until tomatoes begin to wilt, about 5 to 6 minutes.
When cool enough to handle, chop onion into pieces about the same size as tomatoes and finely chop jalapeno. Place all in a bowl along with garlic and vinegar. Just before serving stir in basil and peanuts. Taste and add a little more salt if you wish.
Grease grill and preheat barbecue to medium–high heat. Remove steak from fridge while barbecue is preheating. Season generously with salt and pepper. Place on barbecue and grill about 12 minutes, turning once for medium–rare. Serve sliced with relish on the side.