Make the Walnut Pesto. Place all of the ingredients except the olive oil and water to a food processor. Blend until a paste starts to come together. With the motor running, stream in the olive oil, scraping down the sides as you go, until you have a thick, relatively smooth consistency. Taste for salt and acidity, adding more salt or lemon juice as needed.
Make the Crispy White Beans. Heat a large 12-inch skillet with the 2 teaspoons olive oil, and ensure the entire pan is coated with the oil (I use a paper towel to evenly distribute the oil). Add the rinsed and dried beans in small handfuls and spread them out in a single layer. Let the beans cook undisturbed for 3 minutes. Then use a flat spatula to flip the beans and cook for another 3-5 minutes, or until the beans are browned and blistered. Add the thyme, sesame seeds, salt, lemon juice. Reduce the heat to medium-low and cook for 2-3 minutes to allow the flavors to blend.
Assemble the Sweet Potatoes: Stuff each baked sweet potato with the 2 tablespoons of pesto and 1/4 of the White Beans. If desired, add chopped tomatoes and diced avocado to each potato, and drizzle with a bit of tahini.