When conversation is one-sided, how can I teach turn-taking?

Talking one-sidedly about a favorite topic is also an expression of the child's strength and interest. First, without denying the interest, receive it, then teach the shape of exchange—"talk, listen, take turns"—with visible devices. Pass around an item that shows whose turn it is, make "your turn / my turn" explicit, and practice a pattern for asking the other person ("do you like ○○?"). Build small successes, and separately preparing a time to talk to their heart's content makes switching easier.