Bring a large pot of water to a boil. Meanwhile, in a medium bowl, combine olive oil, lemon juice, red wine vinegar, salt, and pepper. Set aside. In a large bowl, prepare an ice bath. Blanch peas and asparagus in boiling water for 1 minute, then drain and shock in ice bath. Drain, then toss peas and asparagus with tarragon, feta, and dressing. Adjust salt to taste and garnish with red onions.