My first attempt at making a parsnip based bread was from a recipe by Hugh Fearnley-Whittingstall. I had bought a cheap bag of parsnips which were going out of date and needed to use some up since I didn't really want to fill the freezer with parsnip wedges (which would probably have meant I would be eating roast parsnip for several months).
I followed HFW's recipe very closely but used finely chopped rosemary instead of thyme. The substitution worked well. There was one slight problem, which was probably my fault: the dough was a bit too wet and the resulting bread was slightly soggy in the middle. The actual flavour was good and the bread toasted well, which had the advantage of slightly drying out the bread a bit.
My second attempt was a sweet potato and parsnip bread. I started off with the vegetables mashed and left to cool before I added bread flour, yeast and a little water, and mixed to a soft dough. I think the dough was roughly 50% vegetable by weight. I baked the bread in a moderately hot oven until it was brown and crusty on top. The bread was slightly sweet and had a chewy texture but worked really well in a cheese toastie.