# be su:
sudo -i
# list all the packages from the mono repository (into file pkg1):
grep ^Package: /var/lib/apt/lists/download.mono-project.com*_Packages > pkg1
# extract just the package names (into file pkg2):
sed -e 's/^.*Package: //' pkg1 > pkg2
# (optional but I wanted to do that anyway) remove duplicate package names from different architectures and put definitive list of packages to search for and remove in pkg3:
awk '!seen[$0]++' pkg2 > pkg3
# (optional, for check) count the lines in the files (one line = one package) to see what happened:
wc -l pkg1 and wc -l pkg2 and wc -l pkg3
# remove packages listed in pkg3:
apt purge $(cat pkg3)
# cleanup :
rm pkg1 pkg2 pkg3