Preheat a grill for cooking over medium-high heat.
Combine the rhubarb, strawberries, vanilla extract, 2/3 cup sugar, zest of 1 orange and 1/8 teaspoon salt in a large bowl. Lay out 2 large sheets of heavy-duty foil (or doubled layers of regular foil). Divide the rhubarb-strawberry mixture evenly between the 2 sheets of foil. Bring the long ends together and fold twice to seal; fold in the short sides to seal, leaving room for steam. Place the 2 foil packs on the grill. Cook, uncovered, 15 minutes. Remove from the grill and lower the grill temperature to medium. Carefully open the foil packs, making sure to avoid contact with the hot steam. Let cool long enough so the packets are not steaming before serving.
Meanwhile, using an electric mixer, beat together the heavy cream, sour cream, remaining orange zest and 2 tablespoons sugar in a large bowl until thick and fluffy. Keep refrigerated until ready to serve.
Grill both sides of each pound cake slice, 1 to 2 minutes per side, using a metal spatula to flip and remove them.
Serve each grilled pound cake slice with some compote and a dollop of the cream.
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