I learned last night while doing one-legged drills that my left leg is noticeably weaker than my right.
Know any good drills or exercises I can do to build up my scrawny left leg?
The more specific, the better please.

Barbell split squats and stepups are fun.

How about leg presses at the gym. Low weight, high reps to get closer to riding simulation. Do 50 reps with your weaker leg, then 40 reps with your stronger leg. Repeat. That will give your weaker leg some extra workouts. Adjust as required.

Also consult a chiropractor. Imbalances in the spine can cause nerve issues which subsequently cause strength discrepancies between the left and right sides of the body. Before I went to my chiropractor, for instance, my left hand grip strength was 7lbs. weaker than my right; after a few weeks of treatment, my right hand grip strength improved by 3lbs. and my left hand grip strength was only 1lb. weaker than my right.

Strength imbalances aren't necessarily due to one muscle group being smaller than another; nerve control is important to consider as well.

Many thanks for the excellent tips!
As a matter of fact, I had two surgeries last year to cut out a herniated disk = 9 months of no riding and 4 months of not being able to sit in a chair, drive a car or eat in a restaurant. Been doing yoga since then and my back has been much better.
In addition to that, I had a partially-torn ACL and PCL (running is decidedly not as good for the body as cycling) in my left knee the year before that. Since then, I'd been favoring my left leg and it had grown weak.
Thanks again,
-CCinC

This is great advice, if you'd like to ruin your knees and likely you lower discs as well.

One legged squats, split bulgarian squats, even one-legged deads are your best bets.

Some of this depends on what you currently do for your legs. And obviously you have injuries to accommodate.