• Preheat your grill to high heat, letting it heat up for at least 10 minutes.
• For the veggies, heat the olive oil in a large frying pan on medium-high heat. Add in the pepper, onion, celery, and garlic and cook, stirring frequently, until soft and golden brown, about 5-7 minutes. Keep on low heat to keep warm.
• Combine the ground beef, Cajun seasoning, and salt in a large bowl and mix them evenly. Form into 3 3/4 inch patties. Brush each side lightly with olive oil.
• Once the grill is on high heat, put the burgers over direct heat and cook for 4 minutes. Flip and cook an additional 3-4 minutes, or until they reach the desired level of doneness. Transfer to a plate, cover to keep warm and let rest.
• Cut 8 slices out of the middle of the sweet potato (it's the largest part), about 1/4-inch thick. Place them in a large, microwave-safe bowl and add a little water. Cover and microwave until they just begin to soften, about 2 minutes. Drain the water and pat the potatoes dry with a paper towel. You could boil them, too, but it would only be for 2-4 minutes or so.
• Brush each side of the potatoes with the oil and grill until nice char marks form, about 4 minutes. Flip and cook for an additional 3-4 minutes, until fork tender but not mushy.
• While the potatoes cook, mix all the ketchup ingredients in a medium bowl.

To serve:

• Divide the ketchup between all 8 of the sweet potato slices. Divide the veggies between 4 of the slices, and place one burger on top of each veggie-covered slice. Cover with the remaining sweet potatoes.
• DEVOUR!