Original Recipe (from medievalcookery.com):
Source [Ouverture de Cuisine, Daniel Myers (trans.)]: To make May eggs. Take eggs, & put them to boil that they will be soft: then make an hole in the egg at both sides with a point or skewer of wood, that the hole is for putting a pea therein: then blow the yolks out, & chop parsley very finely, & mix it with the egg yolks, a little salt & pepper therein, & put again an egg yolk or two, or more, & refill the eggs with the point of a knife thereon, that it will be layered thereon with a little white bread, that it does not run at all: then have a cauldron of boiling water, & cast the eggs therein, & let them boil hard, afterwards take them out & peel, & cut them in half, & put thereon melted butter & juice of sorrel.
And here's the modern instructions:
1 egg yolk
1 tsp. parsley
1/2 tsp. tarragon
1/2 tsp. salt
1/4 tsp. pepper
1 slice white bread
Place six eggs into a pan of cold water and bring to a boil. After they've been boiling for 3 minutes, remove from heat, and chill rapidly in ice water.
Carefully cut a half inch hole in the shell at the large end of the egg. Cut through the white with the tip of a knife to expose the yolk. Remove the yolk and put it into a bowl (a syringe works well for this). Repeat with remaining eggs.
Mix the six egg yolks together with the extra one, add spices and return to egg shells. Plug the whole in each egg with a small piece of white bread. Place in pan of cold water and bring to a boil. Reduce heat and simmer for 10 minutes.
Remove from heat and chill in ice water for 2 minutes. Then remove the eggs immediately from the water, dry them off, and carefully remove the shell.