Place all of the dressing ingredients except the salt and pepper, in a jar or container with a secure lid and shake it to combine them. Taste and add salt and pepper if desired.
Season the tuna lightly with salt, pepper and red pepper flakes.
Heat a non stick pan with the remaining tablespoon of oil over a high heat.
Using tongs, place the tuna into the pan and sear for about 15-20 seconds.
Turn the tuna over and sear again until all of the sides have been seared.You only want to cook the outside very quickly.
Let the tuna rest while you dress the salad.
Place the salad greens in a bowl and add the orange slices and radishes.
Drizzle about 4 tablespoons of the dressing over the greens and toss gently.
Divide the salad among two serving dishes.
Slice the ahi tuna thinly and place on top of each salad.
Sprinkle the mushrooms over the salad with some of the spiced pepitas.
Lightly spoon some of the dressing over the salad making sure to coat the tuna on top.
Serve at once.