Why You Should Think Twice Before Adding Raw Vegetables To Casseroles - Tasting Table
Briefly

The best casserole recipes require extra effort. Cook vegetables separately to ensure they are thoroughly cooked, avoiding partially cooked bites in the final dish.
Using frozen vegetables can simplify the casserole-making process. Thaw them before adding to prevent texture issues as they release water during the cooking process.
Read at Tasting Table
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