Heat oven. Heat oven to 400°F and arrange a rack in the middle. Place beets in a medium baking dish. 

Blend grape marinade. Blend 2 cups of grapes, with 1 tablespoon of the vinegar, 1 tablespoon of the oil, 1 teaspoon of the salt, and ¼ teaspoon ground black pepper in a blender until smooth. 

Roast beets. Toss beets in the grape marinade. Cover with foil and roast for 30 minutes. Uncover toss to coat in the mixture again and continue roasting and turning every 10 minutes until juices are dry and beets are tender, about 30 minutes more. Let cool. 

Make the dressing. Combine serranos, 2 tablespoons of red wine vinegar, minced shallot, lime juice, white pepper, and ½ teaspoon of the salt in a small bowl. Drizzle in the remaining 4 tablespoons of the olive oil while constantly whisking. Taste and add more salt or vinegar if needed. 

Arrange salad.  In a large bowl, toss fennel with half of the dressing and arrange on a platter. Cut 1 cup of grapes in half and sprinkle on top of the fennel. Top with beets and pistachios and drizzle the remaining dressing over the salad. Garnish with the fennel fronds and serve.