Prune, Orange, Fennel, and Red Onion Salad with Mixed Greens Recipe

1 cup dried pitted prunes
1/2 cup orange juice
1/4 teaspoon fennel seeds
2 oranges
2 tablespoons olive oil
2 tablespoons Sherry wine vinegar
1/2 teaspoon grated orange peel
2 tablespoons minced red onion
2 medium fennel bulbs with fronds; 1 tablespoon fronds chopped, bulbs trimmed and thinly sliced
6 cups mixed baby greens
2 thin red onion rings

Combine first 3 ingredients in small saucepan. Bring to boil. Reduce heat; cover and simmer until prunes are soft and juice is reduced to 2 tablespoons, about 6 minutes. Transfer prunes to bowl; reserve juice. Chill prunes until cold, about 2 hours. Cut into quarters.
Cut peel and white pith from oranges. Working over bowl to catch juice and using small sharp knife, cut between membranes to release segments. Combine 2 tablespoons orange juice from bowl, oil, vinegar, grated orange peel, and reserved prune juice; whisk to blend. Season dressing with salt and pepper.
Gently toss orange segments, minced red onion, 1 tablespoon fennel fronds, and 1 tablespoon dressing in medium bowl. Combine mixed greens, onion rings, and fennel bulbs in large bowl. Toss with remaining dressing. Divide salad among 4 plates. Top with prunes and orange mixture.