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.