Run a microplane kitchen tool across the skins of well-washed and dried feijoas to obtain 2 tablespoons of zest. Combine the zest and lemon juice in a shallow bowl.
Cut the (now zested) fruit in half lengthwise and scoop out the pulp and soft flesh to obtain 1/2 cup then add to the lemon juice-zest mixture.
Use the tines of a fork to mash the fruit with the lemon juice-zest mixture until the consistency of mashed bananas. Work quickly to minimize the fruit oxidizing/turning brown. The lemon juice will help delay the oxidation. The smoother this texture becomes, the smoother the resulting unstrained curd.
Whisk together the lemon juice, zest, pulp, sugar and egg yolks in a 2-quart heavy saucepan. Stir in the butter and cook over medium-low heat, whisking constantly, until the curd is thick enough to hold marks of the whisk when stirred; and the mixture just barely begins to bubble. This should take about 5 - 8 minutes.
Strain through a medium-meshed strainer if desired. Transfer the feijoa curd to a clean bowl. Place a sheet of plastic wrap directly on the surface of the curd and refrigerate until cold, about 1 hour.
The curd can be stored in the refrigerator up to one week.