caramelize
US /ˈkɑːr.məl.aɪz/
UK /ˈkɑːr.məl.aɪz/

1.
to heat sugar until it melts and turns into a thick, golden-brown syrup, used as a flavoring or coloring agent
:
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You need to caramelize the sugar slowly to prevent it from burning.
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The chef decided to caramelize the onions for a richer flavor.