Find a hill that takes you 10 minutes to climb, and do it one or two gears harder than you would otherwise. Cadence can go way down, no worries, and HR should be somewhat below your AT(anaerobic threshold) if you know it (or take a good guess).

I was advised to go as low as 40 rpm (but I ended up going no lower than 60), and perhaps 10 bpm above my "forever" heart rate. That's the "lactate balance point", a heart rate where you feel like you are working, but could go on forever, it's maybe 10-20 beats higher than base endurance and 10-20 lower than AT. For me, that means I do the intervals at max. 160, and my "forever pace" is about 150.

Do that hill 3 times, or do 3x10min on the trainer or spin bike. Do this once a week and work up to 20 min intervals.