In a saucepan, melt 2 tbsp. butter over medium heat. Add the shallot and cook until soft, 5 minutes. Stir in the grapes, sugar and a pinch each salt and pepper. Cook until tender, 5 minutes. Remove from the heat and cover to keep warm.
Preheat the broiler. Meanwhile, in a large bowl, whisk together the eggs, 1/4 cup water, 1/2 tsp. salt and 1/4 tsp. pepper. In a 9-inch, ovenproof nonstick skillet, melt the remaining 1 tbsp. butter over medium heat. Pour in the eggs and cook, stirring constantly, to three-quarters doneness, 4 to 5 minutes. Remove from the heat; spread evenly in the pan; broil until set, 1 to 2 minutes. Dot with the mascarpone.
In a salad bowl, whisk together the EVOO and vinegar; add the spinach, season and toss to coat. Cut the frittato into wedges; top with the grape compote. Serve with the salad.