In a slow cooker, add the onion, orange peppers, chicken breast, fajita seasoning, roasted tomatoes, pepper and onion relish, salt, and pepper. Stir until everything is well combined.
Add the water to the slow cooker. Cook the fajitas on high for 4 hours or on low for 8 hours.
Once cooked, turn off slow cooker and use tongs to place the mixture in a large bowl.
For the queso
In a medium saucepan, melt the butter over medium heat. Add the flour and continue to whisk until a roux forms.
Gradually pour in the milk and continue to stir until slightly thickened, about 5 minutes.
Remove saucepan from heat and stir in cheese, pepper and onion relish, salt, and pepper.
To assemble the subs
Take a sub and dig out the bread. Place some of the fajita mix inside until it's comfortably full.
Top the sub with the queso relish and garnish with desired toppings. Repeat until all subs have been assembled.