Prepare grill. Over high heat, grill hot dogs until lightly browned. Place buns, cut side down, on cooler part of grill until lightly toasted for about 3 minutes.
Split avocados into two, pit, peel and crush. Gently mix Meshed avocado and lemon juice in a medium-sized bowl. Season with salt to taste.
To serve, place one hot dog in each toasted bun. Evenly divide diced tomatoes over hot dogs and top with avocado mixture. Sprinkle with optional thinly sliced onion and serve.