Peel and halve yellow onion (julienne). Slice into thin strips. Mince garlic. Cut end off romaine, halve lengthwise, then coarsely chop. Halve grape tomatoes. Coarsely chop roast beef into 1" pieces.

Heat 1 tsp. olive oil in a medium pan over medium heat. Add garlic and cook 2 minutes. Add evaporated milk and simmer 5 minutes. Whisk in cheddar cheese (reserving 1/4 for pizza topping) and simmer 2 minutes until sauce is thick enough to coat the back of a spoon and adhere to pizza. Season with a pinch of salt and pepper.

Spread cheese sauce on top of naan flatbreads. Evenly divide roast beef between both, then top each with reserved cheddar cheese. Place assembled pizzas in oven directly on oven rack. Place baking sheet on the rack just below to catch any drippings, and bake 10-12 minutes. Wipe pan with cheese sauce clean.

While pizza cooks, heat 1 tsp. olive oil in pan used for cheese sauce over medium-high heat. Add onions and cook about 5 minutes, stirring constantly until onions start to slightly char. Add 1/4 cup water to pan and cook, while stirring, until evaporated, about 5 minutes. In a large mixing bowl, combine sugar, white wine vinegar, 2 tsp. olive oil, half the red pepper flakes (start with a small amount, taste, and add more if desired), and a pinch of salt. Whisk together thoroughly and set aside.

Remove pizzas and baking sheet from oven, turn broiler on high, and transfer pizzas to baking sheet (careful, it will be hot) and into broiler 1 minute until cheese becomes bubbly. Remove from oven and top with caramelized onions.

Toss romaine, grape tomatoes, and vinaigrette together and divide between two plates. Slice each pizza, garnish with remaining red pepper flakes (to taste), and place alongside salad.