Heat the oil in a non-stick pan over medium heat. Add the diced onions and ham, and cook for 4-5 minutes, stirring occasionally, until the onions soften.
Add the spaetzle and cook for 2-3 minutes, stirring gently to combine.
Pour in the milk and bring to a boil over medium-high heat, about 3-5 minutes, stirring occasionally.
Add the processed cheese slices and stir constantly until melted and the sauce reaches your desired thickness, about 1-2 minutes.