Preheat oven to 325°. Mix salt, paprika and pepper; sprinkle over ribs. Refrigerate, covered, while preparing sauce.
In a large saucepan, combine rhubarb and strawberries; add water to cover. Bring to a boil. Cook, uncovered, until rhubarb is tender, 8-10 minutes. Drain; return to pan. Mash until blended.
In a Dutch oven, heat 1 tablespoon oil over medium heat. Brown ribs in batches, adding more oil as needed. Remove from pan.
Add onion to same pan; cook and stir until tender, 4-6 minutes. Add remaining ingredients; stir in rhubarb mixture. Return ribs to pan, turning to coat. Bring to a boil. Cover and bake until ribs are tender, about 2 hours. Bake, uncovered, until sauce is slightly thickened, 30-35 minutes.