Rinse quinoa in cold water. Heat saucepan with drizzle of olive oil. Roast quinoa for a minute. Stir in 1.5 C water and bring to a boil. Reduce heat, cover, and simmer 15 minutes. Let stand 5 minutes covered.
Preheat oven to 375F.
Slice avocado in half lengthwise, and scoop out innards (preserving the skin). Gently mix together the quinoa, avacado, and tomatoes.
Fill each avocado shell with the quinoa mixture. Top with crumbled feta cheese. Bake in oven 10-12 minutes. Serve immediately.