what does this phrase said by TouchStone means : 'Of a certain knight that swore by his honour they were good pancakes and swore by his honour the mustard was naught'?

translation;

From a certain knight who swore on his honor that the pancakes he was eating were good, and that the mustard on top of them—he swore on his honor—was bad. Now, I’ll swear to the opposite—that the pancakes were rotten, and the mustard was fine—but even so, the knight wasn’t lying.