It turned out to be pretty good, although it was quite time consuming to process the pears. (If anyone has tips on processing prickly pears, I'd love to hear them.) And next time it will take even longer, because I'll use more prickly pears.
prickly pear mousse
5 prickly pears
2 C heavy cream, chilled
2 egg whites
1/8-1/4 C sugar
Peel the prickly pears, using caution and/or gloves. Even though most of the spines are removed from those sold in stores, I still managed to get on in my hand, and because it was so tiny, it was hard to find and remove.
Mash through a strainer, saving juice and discarding seeds. I used my pestle with the strainer for this.
Reduce juice (I started with around 1.5 C) to around 1/4 C. (I reduced mine in two batches; the first I reduced too far--it was a thick paste--and the second I left more watery. Next time, I'll shoot for a nice syrupy reduction.)
Whip cream to stiff peaks, adding half of sugar when soft peaks are reached. Mix in prickly pear reduction. Whip egg whites to stiff peaks, again adding sugar when soft peaks are reached. Lightly fold into whipped cream mixture.
We ate our mousse immediately; I don't know how well it keeps.