Peel skin from kohlrabi and potatoes, dice to desired size.
Place kohlrabi, potatoes, and parsley in bowl and toss with olive oil, salt, and pepper.
Line sheet pan with parchment paper. Place kohlrabi mixture on sheet pan in a single layer.
Place in oven and bake for 10 minutes, flip and bake for another 10 minutes or until kohlrabi and potatoes are soft, yet browned.
Peel the onions and slice them very thinly.
Heat the butter over medium heat in large skillet. Saute onions for 20-30 minutes, stirring every few minutes. Onions should be deeply golden brown and fragrant. Remove from heat and allow to cool.