[Chris15]

Inductance.
Lots of it.

Since the voltage a cross an inductor is equal to L di/dt, the current through an inductor must remain continuous. It can't jump suddenly. This is why they are used on the outputs of phase chopping dimmers, to smoothe the curve...

Inductor goes in series with the load. You may need to add a parallel or series capacitance to return the whole thing to having a reasonable power factor.. The value and configuration will require some analysis but it's easily enough achieved...