Place 1 Tbsp. of oil in a pot set over medium heat. Add onion, green pepper, celery and garlic and cook 3 minutes. Mix in flour and cook 1 minute more. Slowly, while stirring, mix in stock. Add the tomatoes, chilies, sugar and thyme. Bring to a slow simmer and simmer 5 minutes. Season with salt and pepper, and then cover and reserve sauce on low heat.
Preheat grill to high. Brush tuna with the remaining oil; season with salt and pepper. Lightly oil the bars of the grill. Grill tuna on all sides for about 20 seconds per side, until seared on the outside, but still rare in the middle. Set tuna on a cutting board and cut into 1" thick
pieces. Divide the sauce among 4 plates. Divide and set the fish on the sauce and serve.