In a large saucepan or pot filled with salted water, boil the sliced potatoes until they just become tender and are easily pierced with a fork. Then drain potatoes from the water and set aside.
While the potatoes are cooking, add oil to a saute pan set over medium high heat and add the chopped peppers, onion, and sliced kielbasa. Saute the sausage mixture until the vegetables become tender and the kielbasa is nicely seared.
Reduce the stovetop to medium heat and add the fresh spinach. Stir it into the vegetable mixture until it begins to wilt, then add the cooked potatoes.
Prepare the mustard sauce by adding the heavy cream, mustard, garlic powder, fresh thyme, fresh ground pepper, and salt to the saute pan. Stir everything to combine, then bring the sauce to a simmer.
Continue to simmer for 2-3 minutes, or until the cream sauce thickens. Then serve immediately.
Notes
If you wish to make this as a one-pan skillet, the potatoes can be pan-fried prior to sauting the sausage and vegetables.