1. Rinse the onions under cold water, then drain. Toss the onions with 2 tablespoons lime juice, the oil, 1 teaspoon salt and 1/2 teaspoon pepper in a large bowl. Let stand until softened, 5 minutes.
2. In another bowl, whisk together the soy sauce, sugar, jalapenos and the remaining 4 tablespoons lime juice. Stir in the chopped herbs. Add the portobellos and half the sesame seeds and gently mix until well coated with the marinade. Let stand until softened, about 6 minutes, tossing occasionally to better incorporate the marinade.
3. Transfer half the onions to the bowl with the mushrooms. Toss the spinach in the large bowl with the remaining onions. Transfer to a platter, top with the marinated mushrooms, the remaining sesame seeds and pickled onions. Drizzle with the mushroom marinade and serve.
Always rinse spinach well to remove the sand that lurks in its creases. Fill a big bowl with cold water. Add the spinach, swish it gently with your hands and wait a minute to let the grit settle to the bottom. Lift the spinach from the water, leaving the grit behind. Dry the leaves in a salad spinner or gently roll between 2 clean kitchen towels.