Preheat oven to 375°F (190°C). Prepare a pouch by folding a 12- by 24-inch sheet of aluminum foil in half and crimping two edges to form a pocket.
Toss beets with 1 tbsp olive oil, thyme or rosemary (if using), and a pinch of salt and pepper. Place in the foil pouch, seal, and set on a rimmed baking sheet. Roast for about 1 1/2 hours, or until a toothpick easily pierces the beets. Remove from the oven and let cool.
Once cool, peel the beets by rubbing the skins under cold running water. Cut into 1 1/2-inch chunks. (Beets can be stored in the refrigerator for up to 5 days if preparing in advance.)
Lightly crush the pistachios using a mortar and pestle or by chopping with a knife. Reserve half for garnish.
In a large bowl, combine the grapefruit juice, orange juice, lemon juice, shallot, herbs, and honey. Whisk to blend, then slowly add the remaining 3 tbsp olive oil, whisking continuously to emulsify. Season with salt and pepper to taste.
To serve, toss the beets and citrus wedges with the pistachio vinaigrette. (If using red beets, toss separately to prevent staining other ingredients.) Spread half of the ricotta on a serving platter, top with the dressed beets and citrus, and dollop with the remaining ricotta. Garnish with reserved pistachios and serve.