Spray a pan with oil spray; sauté 1½ cups (150g) chopped mushrooms, ¼ onion, and 2 cloves of chopped garlic over low to medium heat for 2 to 3 minutes.
In a small bowl, mix tahini with lemon and water until smooth and well combined; set aside.
Add chopped ahi tuna (you can buy it frozen and thaw it in the fridge).