Where or how is a good place to learn to start on a hill? Example getting caught in a underpass with a steep grade.

As far as where to practice, I'm unsure but...

The basic principle is to keep your foot on the back brake only, put it in 1st, slowly release the clutch until you find the friction point (where it engages) and then let off the rear brake as the clutch begins to hold the bike and then move off.

Practice finding your clutch friction point on level ground first. Don't rev the bike at all, just let the clutch out with both brakes off until it starts to move slightly and then pull the clutch in again. Knowing your bikes friction point will help immensely with starting on a hill.

And welcome to SMA by the way!

Practice for me was getting to the top of the university bridge in the right lane and being first in line, steeper than I thought.
It's just like riding a bike, which surprised me a little.

I would find a steep driveway on a quiet commercial street on Sunday.

I think I'm going to hit a loading dock at a large store or warehouse area. Should give me the environment to practice in a safe manner.

That's a good idea