Place onions, bell pepper, tarragon and mayonnaise in a mixing bowl and combine. Gently fold in the crabmeat. Add the panko and season with salt, white pepper and cayenne pepper to taste. Let the mixture rest for 5 minutes. The panko will absorb some of the moisture and the mixture will stay together.

Lay out a large piece of plastic wrap on a clean counter surface. Place half of the crab mixture on it and use a spatula or a spoon to form it into a tube about 1 3/4 inches in diameter. Bring the wrap up over the crab and roll the crab mixture up. Twist the ends to close. Pierce any air pockets with a toothpick or skewer. Twist the ends even tighter to compress the crab mixture. Tuck under the ends of the wrap and place the tube on a plate. Repeat with the second half of the crab cake mixture. Place the tubes in the refrigerator for at least 1 hour or overnight.

When ready to cook, Preheat oven to 350 degrees. 
Cut the tubes of crabmeat into 1 1/4-inch-thick cakes. Gently remove the plastic wrap, leaving the cakes in nice cylinders.

Heat 3 tablespoons oil in a heavy-bottomed frying pan (non-stick works well here) over medium-high heat until very hot, but not smoking. Gently place 4 of the crab cakes in the pan and sear for 3 to 4 minutes or until golden brown. Gently turn the cakes over and sear the other side until golden brown. Place them on a baking sheet and into the oven to keep warm. Wipe the pan, add the remaining olive oil and repeat this process for the other 4 crab cakes. (If you have a big enough pan, you can cook 6 or 8 of them at the same time and skip the warming oven, but the cakes need room to sear properly and be turned so be careful not to crowd the pan)