I’d like to eliminate dairy and most of the salt. If there’s not a substitute, what would be a good final touch?
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They do make vegetarian/vegan sour cream, using soy milk. I imagine you could also use a soy yogurt, draining it if needed to get the texture you want. I can't read the recipe you posted, so I can't say if eliminating the salt would be a good idea or not. If you just want to limit your sodium intake, you could use a salt substitute, or even Lite Salt, which is a half/half blend of regular and salt substitute. It will react essentially the same chemically in a recipe, but let you cut the regular salt. However, sometimes you can be left with a metallic taste, especially if the salt is used on top, like with pretzels.
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I also couldn't read the photographed recipe.
Is the creme fraiche mixed in or used as garnish/topping?
If mixed in, I have little to add to Lori Terwilliger's suggestions of vegan sour cream (soy, coconut, various). Possibly some homemade or no-added-salt nut butter, diluted with a little water, to help smooth & finish a sauce.
If a garnish, maybe vegan ice cream or non-dairy sorbet.