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.