Peppery watercress has a high amount of dietary nitrate—which has been shown to lower blood pressure. And fiber-rich mango gives a nice boost of vitamin C.
Bring a pot of water to a boil over high; add green beans, and cook until just tender, about 3 minutes. Drain; rinse under cold water for 30 seconds. Set aside.
Heat 1 tablespoon of the oil in a large nonstick skillet over medium-high. Sprinkle salmon evenly with black pepper and 1/2 teaspoon of the salt. Add salmon to skillet, skin side down; cook to desired degree of doneness, about 4 minutes per side for medium. Remove from heat.
Whisk together lime juice, honey, Fresno chile, and remaining 2 tablespoons oil and 1/2 teaspoon salt in a small bowl. Toss together watercress, mango, onion, mint, cilantro, and green beans in a separate bowl; add half of the lime juice dressing, and toss. Sprinkle with peanuts. Serve with salmon and remaining dressing.