You can't mix bases, oil based finishes can be mixed, water based finishes can be mixed, oil based finishes cannot be mixed with water based finishes. PU varnish is oil based so it should go over lard once its cured. It certainly goes over oil paints, tung, linseed, flax oils. Try a tiny patch test and check but it should be ok. Check your varnish is clean with thinners or white spirits (oil base) not water and soap .