I do it using the local gcc-arm build chain:
cd firmware/main
make all PLATFORM=xenon MODULAR=n APPDIR="/path/to/source"
It should be possible from Particle Workbench but I believe you need to modify the tasks.json file because there isn’t a build task for building a monolithic binary.
If switching between modular and monolithic, probably do make clean all instead of just make all.