godan verb (-su)
boil and discard the liquid
Cooking technique where ingredients are boiled and the water is thrown away, often to remove bitterness, astringency, or scum.
たけのこは茹でこぼしてあくを抜きます。
Boil the bamboo shoots and discard the water to remove the bitterness.
Standard kanji-kana mixed spelling.
Rare kanji spelling using 溢す; not commonly used.
茹でる simply means 'to boil'; 茹でこぼす specifically includes discarding the boiling liquid.
Compound of 茹で (yude, 'boiling') + こぼす (kobosu, 'to spill, discard').