Mestoklema tuberosum is a plant i tried on a whim a few years ago. It looks like an upright ice plant (delosperma) and so my results worked out. The plants are almost everblooming even in winter. My plant has taken -8F with light snow cover and -2F with no snow cover. It died to the ground in 2020-2021 winter when they took -17F. Plants have a large tuber that they came back the following spring. I took cuttings off this plant last fall and they rooted over winter in the greenhouse. I would suggest this plant for the more experienced grower or the ones who like to risk their plants.
I do not guarantee these in a zone 5/6/7 because everyones growing conditions are different but this would be a good trial plant to grow in your cold outdoor garden. Try to plant outdoor 90 days before 1st hard freeze.