Don't thaw the veggies: I put them into the casserole straight from the freezer. That first 30 to 35 minutes in the oven heats them through before the sauce goes on.
Stir them halfway through: Don't skip this little step. It helps the frozen vegetables heat more evenly so you don't end up with cold spots.
Melt the sauce gently: Keep the saucepan over medium heat and stir until the Velveeta is completely melted and the sauce is smooth. There's no need to rush it with high heat.
Fresh mushrooms are my pick: Canned mushrooms are convenient, but I prefer fresh when I have them because they give the casserole a little more texture.
Want more crunch?: I like regular breadcrumbs for a light topping, but panko or crushed buttery crackers are great if you want the top noticeably crunchier.
Look for bubbling edges: The casserole is ready when the sauce is bubbling and the edges are beginning to brown. Keep an eye on the breadcrumbs toward the end so they don't get too dark.
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