If shrimp are frozen, place them in a colander and run cool water over them. Peel and devein if needed.
Meanwhile, make the dressing by adding all the dressing ingredients except for the olive oil to a food processor. Blend on high while slowly drizzling in olive oil. You may need to scrape down the sides and blend again until the mixture is smooth. Set dressing aside (you can refrigerate it if you wish).
Prep the other ingredients (spinach, mushrooms, cucumber, avocado, tomato) and place them in a large salad bowl.
Add a bit of olive oil to the bottom of a skillet (1-2 teaspoons) and the juice from the other half of the lime. Turn heat to medium/medium high, add shrimp, garlic powder, and salt and pepper. Cook for a few minutes until shrimp are pink and cooked through.
Add shrimp to salad and toss with dressing. Serve immediately.