Boil water on the stove in a medium saucepan. 

Crumble both cubes into the water and mix to dissolve. 

In a small saucepan, melt butter over medium heat. 

Add flour, onion powder, and pepper to the butter. Stir with a whisk. 

While stirring, slowly pour in half of the water mixture. As you mix the water into the flour, it will turn into a roux. Do not stop mixing. It will thicken quickly. Add the remaining water and mix. 

Stir for 1-2 mintues or until the mixture thickens to the consistency you want. 

Taste and season as desired.