Preheat oven to 425°F. Toss sweet potatoes with 2 Tbsp. of the oil and 1/2 tsp. of the salt; arrange in a single layer on a baking sheet. Bake for 30 to 40 minutes, tossing once halfway through, under tender and slightly caramelized.
While potatoes roast, prepare sesame dressing by whisking remaining 2 Tbsp. oil with tahini, 2 tsp. of the lemon juice, ginger, fish sauce, and sugar until smooth. Gradually whisk in up to 2 Tbsp. water (in 2 to 3 tsp. increments) until sauce is smooth and reaches desired level of creaminess. Set aside.
In a large bowl, combine radicchio and parsley. Toss with remaining 2 tsp. lemon juice and 1/2 tsp. salt. Add roasted sweet potatoes and gently toss all ingredients with sesame dressing. Garnish with sesame seeds, lemon wedges, and additional fresh parsley, if desired.
If making vegan, swap fish sauce for either soy sauce or tamari.