Warm butter and oil in a large skillet, and add onion, shallot and garlic, cooking until tender. When starting to caramelize, add in heavy cream and cream cheese, stirring slowly until combined.
As liquid all warms, add broth (or water) gradually to stretch sauce without diluting too much. Add cubed swiss and mustard, continuing to stir slowly.
The sauce should form a nice alfredo consistency. Use a little more cream or water to form preferred thickness. Personal preference, I like a little more sauce to greens ratio, as I used the excess elsewhere.
Clean radish greens and slice in fairly wide strips, adding to warmed cheese sauce. Fold to combine.
Once radish greens are wilted, add in handfuls of spinach, combining carefully. Allow mixture to slowly simmer for about 5 minutes, allowing all flavors to build and greens to wilt.
Option here to serve immediately, or divide into individual ramekins and keep warm in the oven. For this plate, I used the excess cheese as a sauce over a chicken breast with portabella mushrooms as my main dish, serving a ramekin of greens as the side, as shown in photo.