Zest Lime (1) into a small bowl and set aside. Juice lime into a medium bowl and set aside. Peel and finely chop Fresh Ginger (0.25 ounce) and add to lime juice.
Add Fish Sauce (2 teaspoon), Granulated Sugar (1 tablespoon), Salt (1/4 teaspoon) and Ground Black Pepper (1/4 teaspoon). Slowly whisk in Olive Oil (1/4 cup) until fully incorporated. Place half of the marinade into the refrigerator and pour remaining into a large bowl. Set aside.
Finely chop Garlic (1 clove) add to large bowl with marinade. Cut half of the Red Fresno Chili Pepper (0.5 ounce) in half, widthwise. Cut into quarter inch sliced rings and place into a second large bowl.
Remove the stems from the Fresh Cilantro (0.5 ounce) leaves and set stems aside. Roughly chop leaves and place in the large bowl with the chili rings. Slice the Cherry Tomato (5 ounce) in half and add to the chili and cilantro leaves.
Whisk the lime zest, chopped fresno chile and cilantro stems into the large bowl with the marinade. Pat dry Ahi Tuna Steak (2) with paper towels and place the tuna into the large bowl with the marinade. Coat the ahi and let marinate.
Heat a medium saute pan over medium high heat with Vegetable Oil (1 tablespoon). When oil is hot, remove the ahi tuna from the marinade and discard marinade. Carefully add the tuna to the saute pan and sear on each side for one minute. Remove from heat.
Remove two inches of the Watercress (1 bunch) stems and discard stems. Add to the bowl with the chilies, cilantro, and cherry tomatoes. Remove the marinade from the refrigerator and mix to combine. Add to the watercress salad.
Season salad with Salt (1/4 teaspoon) and Ground Black Pepper (1/4 teaspoon). Gently toss.
In the center of two plates, place the watercress salad. Top with seared ahi tuna.