Stir together mayo and lemon juice. Season with salt and pepper, and refrigerate until needed.
Place walleye, onion, chiles, egg yolk and tarragon in a blender and pulse to combine. The mixture shouldn't be perfectly smooth, but should come together easily. Form into about 8 (2-inch) patties. Refrigerate walleye cakes for at least 20 minutes. Dredge chilled cakes in panko and refrigerate for at least another 15 minutes.
Add vegetable oil to a skillet over medium-high heat. When oil is shimmering, add 4 fish cakes and cook until golden brown, about 2 to 3 minutes. Flip and cook for an additional 2 to 3 minutes. Drain cakes on a paper-towel-lined plate. Repeat until all the cakes have been cooked. Serve with lemon mayo.
Walleye Cakes (The Minnesota Star Tribune)