Season salmon fillets with 3/4 tsp. salt and 1/4 tsp. pepper. Cook salmon, covered, in 1 Tbsp. hot olive oil in a large skillet over medium heat 8 to 10 minutes on each side or until done.
Cut onion into 1/4-inch-thick slices. Brush with 2 tsp. olive oil, and sprinkle with 1/4 tsp. salt and 1/4 tsp. pepper. Heat a grill pan over medium-high heat; cook onion slices 4 to 5 minutes on each side or until lightly charred and tender.
Arrange arugula on a serving platter; top with onions, salmon, avocados, and next 3 ingredients. Drizzle with Yogurt-Basil Vinaigrette.