How to Make It
Preheat oven to 400°F.
Combine stock, vinegar, 2 tablespoons sugar, half of the rhubarb, and half of the strawberries in a saucepan. Cover and bring to a boil over high. Reduce heat to medium-low; cook 15 minutes. Pour mixture through a fine wire-mesh strainer into a bowl; discard solids. Reserve liquid.
Heat 2 tablespoons olive oil in a heavy roasting pan over medium-high. Rub remaining 1 tablespoon oil over lamb; sprinkle lamb with oregano, thyme, 1 1/2 teaspoons salt, and 1 teaspoon pepper. Place lamb in pan; cook, turning occasionally, until browned on all sides, 8 to 10 minutes.
Scatter fennel around lamb in a single layer. Transfer to preheated oven; roast until a thermometer inserted in thickest portion of lamb registers 130°F and fennel is caramelized and tender, about 20 minutes. Using tongs, transfer lamb to a cutting board; place fennel on a platter, and cover with aluminum foil to keep warm. Reserve 2 tablespoons pan juices in roasting pan; discard remaining pan juices.
Add remaining rhubarb and strawberries to roasting pan; toss to coat. Sprinkle with 1/4 teaspoon salt and remaining 2 tablespoons sugar; arrange in a single layer. Roast at 400°F until just tender, 4 to 5 minutes. (Do not overcook.) Transfer to platter with fennel.
Remove roasting pan from oven; add stock mixture. Cook over high, stirring occasionally, until reduced to about 3/4 cup; stir in remaining 1/4 teaspoon salt and remaining 1/4 teaspoon pepper. Carve lamb; add to platter. Drizzle with 1/4 cup sauce; top with fennel fronds, if desired. Serve with remaining sauce.