Begin by making cheese crisps that will become crunchy topping. Heat non-stick pan over medium heat. Sprinkle tablespoons of shredded cheddar onto hot pan, leaving room between each addition. Cheese will melt and become bubbly. As cheese melts and becomes oily, gently tilt pan so oil runs off. Use paper towel to blot away the excess oil. (Note: It may not look pretty at this stage, but it will be divine on the finished mac and cheese!) When cheese begins to harden and stick together, flip over with spatula. Cheese will become lacy and cohesive, holding together and no longer being bubbly; remove to cooling rack and sprinkle with garlic salt while still hot. Cheese crisps will harden to cracker-like consistency upon cooling. Once completely cool, chop/break into small pieces to make crunchy crumble topping.
Cook pasta according to package directions; drain and set aside.
In heavy saucepan, melt butter and add diced onions. Cook on medium-low heat until onions are soft and translucent. Once onions are cooked through, add flour. Cook 3 minutes, stirring constantly.
Add milk slowly, whisk constantly. When milk is fully incorporated and mixture begins to thicken, reduce heat to low and add remaining cheese. Continue to stir as cheese melts. Add paprika, salt and pepper, and continue to stir.
Add cooked pasta, stir well until blended. Serve immediately, top with the cheese crisp crumbles.