Heat half a tablespoon of butter in a small skillet over medium heat. Add in the onions and cook for 20 minutes, stirring occasionally, until they are caramelized. Add in the spinach and cook for 2-3 minutes or until it has wilted. Set aside.
Heat the remaining half tablespoon of butter in a large skillet. Add the chicken breasts and cook for 6-7 minutes per side or until golden brown with an internal temperature of 170 degrees.
Meanwhile, heat the butter for the sauce in a small saucepan. Stir in the flour and cook for 2 minutes.
Add in 1/4 cup of the milk and stir quickly. This should make a thick paste. Slowly add the remaining milk, stirring constantly, until it is incorporated into the flour mixture. Cook for about 5 minutes or until the sauce has thickened.
Turn off the heat and add in the cheese, salt, and pepper. Stir until completely combined.
Divide the couscous between three plates. Divide the caramelized onions and spinach and place on top of the couscous. Top each pile of couscous with a sauteed chicken breast and then pour a spoonful of the Dubliner sauce on top.