3 cups low-sodium chicken broth
1/2 medium red onion, very thinly sliced
2 cups cherry tomatoes, cut in half lengthwise, divided
1/2 pound small potatoes, preferably fingerling and purple, sliced into 1/4-inch rounds
3 sprigs basil, plus leaves for garnish
1/4 teaspoon red-pepper flakes
Coarse salt and freshly ground pepper
4 skinless cod fillets (4 ounces each)
4 ounces snap peas, trimmed and thinly sliced on the bias
1 teaspoon fresh lemon juice, plus 4 lemon wedges, for serving
Extra-virgin olive oil, for drizzling
In a large saute pan with a tight-fitting lid, bring broth, onion, 1 1/2 cups tomatoes, potatoes, basil sprigs, red-pepper flakes, and 2 teaspoons salt to a boil over high heat. Reduce heat and simmer, uncovered, until potatoes are crisp-tender, 6 to 8 minutes. Season cod with salt and pepper, add to broth mixture, and cover, simmering about 7 minutes more.
Remove and discard basil sprigs. Add peas, remaining 1/2 cup tomatoes, and lemon juice to skillet, gently stirring to combine; cook just until warmed through. Divide fish, vegetables, and broth among 4 bowls. Garnish with basil leaves, drizzle with olive oil, and serve with lemon wedges.