Feijoa is a type of guava that is bred to grow in colder climates (like northern CA), as opposed to the tropical climates that guavas normally grow in. It's also called a "pineapple guava." We have one in our backyard. The fruit tastes like a blend of pineapple and strawberry. I think it is very tasty. I eat it raw and I make jelly out of it. Unfortunately, I haven't had any success cooking with it, in either desserts or savory dishes. The fruit has sort of a gritty texture when it is cooked and, although the flavor is nice, the texture ruins it for me.
I have never seen Feijoa available commercially or even at my local farmers' market, despite the fact that the trees are quite common and do well here. But you could probably substitute regular guavas--which I don't think would have the same grittiness.
I would be curious to know what the recipe is. We get a TON of guavas off our tree--I make so much jelly I have trouble getting rid of it all. Seriously, I put up 100 half-pints of jelly this year. I would love something else to do with the guavas besides making jelly.