Harrison (in Edible Native Plants of the Rocky Mountains) and Euell Gibbons (in Stalking the Faraway Places) report eating Caltha leptosepala both raw and cooked, and give favorable reports of it. Sam Thayer reports on Caltha palustris in the Forager's Harvest. The consensus is that the white western species is much better tasting than the yellow eastern species.
The sources recommend picking the young leaves in the early spring before it flowers. I collected mine at the beginning of June, between 9000 and 10000 feet elevation, next to melting snowbanks. Spring comes late in the high mountains. The first flowers were already blooming, and comparing different plants, it seems to me that the flowers appear as soon as they can, with little time between sprouting and flowering. Some sources mention using roots, but the roots of these are small and stringy, so not worth even trying anything with them.
Marsh Marigolds have a toxin or two (with various names; look it up and pick one) which is destroyed by heat, so most sources recommend cooking it. Most scientific papers are specifically about Caltha palustris. I don't find good sources for the constituents of Caltha leptosepala specifically. Since it is closely related to C. palustris, we would expect to have similar constituents, but from the reports of less bitterness, it might have less of them.
I tasted it raw, and found the young leaves to be slightly bitter, and the older leaves to be definitely bitter. I'm not sure how early in the spring one would have to find them to get non-bitter raw leaves, but I would not choose to eat them raw. The texture was a bit tough, but very acceptable for a leaf. (Sam Thayer describes the flavor of Caltha palustris as "acrid" but I would not use that word for this plant.) After cooking for five minutes, they were tender but still bitter. At ten minutes they were mushy but becoming less bitter. After a change of water and a total of 15 minutes, most of the bitterness was gone. And after twenty minutes they were about the same mushiness but with still a hint of bitterness.
After the full cook time, the flavor was acceptable. Bland, mushy, and only a hint of bitterness. But so bland that the bitter hint is the most noticeable flavor. The bitter aftertaste seem to stay in the mouth for quite a while after eating them.
Sources recommend eating them in a cream sauce. That would probably help mask the bitterness. For that I will refer you to Wild Food Girl, who makes "creamed elkslip" out of Caltha leptosepala. Elkslip is another common name for Caltha species.
As for myself, I did not find the flavor to be compelling enough for me to want to eat more of it. There was Spring Beauty (Claytonia lanceolata) and Glacier Lilies (Erythronium grandiflorum) growing in abundance at the same place and time, and both of these are much better greens, and both have very tasty roots as well.
