This salad is ideal for cold grilled steak, as reheating may overcook the meat or cause the basil and arugula to wilt. Grilling the red onion adds another layer of char and a bit of sweetness to counter the tartness of the vinaigrette.
Heat a grill pan over high heat. Coat pan with cooking spray. Add onion slices to pan; grill 10 minutes or until tender, turning occasionally. Place onion in a bowl; cover with plastic wrap. Let stand 10 minutes.
Combine oil, vinegar, salt, and pepper in a large bowl, stirring with a whisk. Add onion slices and any accumulated juices to oil mixture. Add tomatoes; toss gently to coat. Let stand 15 minutes, stirring occasionally. Stir in arugula.
Divide tomato mixture evenly among 4 plates; top evenly with steak and avocado. Sprinkle with basil and almonds.