Preheat the oven to 350º. Wrap each sweet potato in foil and place on a baking sheet. Roast until tender, 1 hour.
Meanwhile, make the lemon ricotta: In a medium bowl, whisk all of the lemon ricotta ingredients together until smooth.
Let cool slightly, then peel each sweet potato and rice through a potato ricer into a large bowl. Add the ricotta, eggs, flour, salt and pepper, and stir with a wooden spoon until a dough forms.
Transfer to a floured work surface and knead until smooth. Your dough will be soft. Cut the dough into 4 pieces and roll each into a long rope, ½ inch thick. Using a bench scraper or paring knife, cut the gnocchi into ½-inch pieces. (If you want, you can press the gnocchi against a gnocchi board.) 5. Bring a pot of salted water to a boil. Once boiling, heat the butter with the garlic, thyme and sage in a large skillet over medium-high heat. Cook, stirring occasionally until the butter has browned, 4 to 5 minutes. After 2 minutes of cooking the butter, drop the gnocchi into the boiling water and cook until they float, 2 minutes.
Using a slotted spoon, transfer the gnocchi to the pan of brown butter and cook, stirring often, until lightly browned, 2 to 3 minutes. Remove from the heat and season with salt and pepper. Divide between plates and top each with a dollop of the lemon ricotta, then serve.