Wash tomatoes, halve crosswise and remove seeds. Sprinkle cut portion with salt and sugar. In large skillet over medium-high heat, or on hot grill, cook uncut side in skillet for 1 minute. Remove tomatoes from pan or grill. Fill tomato halves with potato mixture. Cook, potato side down, in skillet or on grill just until potato is lightly browned, about 3 to 5 minutes.