Place red onion, lemon juice, kosher salt, and sugar in a small bowl. Allow to stand one hour, tossing occasionally.

Soak skirt steak in water for 15 minutes to remove excess salt. Combine olive oil, garlic, Worcestershire sauce, and pepper in a bowl. Add the skirt steak and marinate for 30 minutes to an hour.

Heat a large frying pan with a little oil. Add the skirt steak pieces once the pan is hot. Sear for three minutes each side or until cooked to desired temperature. Allow to sit for 10 minutes and slice against the grain.

Place sliced mushrooms in a medium-sized saucepan with olive oil. Sprinkle with salt and pepper. Brown over medium-high heat, stirring occasionally. After five minutes, add the sherry cooking wine. Raise the heat and stir until all the liquid is absorbed. Remove mushrooms from heat. Cool.

Place all pesto ingredients in a blender. Puree until completely smooth.

Prepare six salad plates. You can either mix all ingredients in a large salad bowl and toss with the desired amount of pesto, then divide evenly among the salad plates; or you can layer the salad ingredients on each plate and top each plate with a dollop of pesto.