You should not thaw frozen vegetables before cooking them.
While you can thaw out frozen vegetables before cooking them, it's best not too and there's really no need to thaw them out.
It's best to cook the frozen vegetables straight from the freezer.
You can simply add the frozen vegetables straight from the freezer to pastas, stews, soups, baked foods etc.
Although for some dishes such as dips and salads, it's best to thaw the frozen vegetables out and drain them to avoid excess moisture.
But for other dishes that are cooked it's best to add the frozen vegetables without thawing them and then cook them from frozen.
The heat from the cooking process will thaw out the frozen vegetables and cook them.
You can also roast or stir fry frozen vegetables directly from frozen although some frozen vegetables may become mushy if they're overcooked.
And corn, peas and other small vegetables will cook quickly and evenly and can be added directly to the cooking process.
However leafy greens like spinach should be thawed if frozen before cooking, to drain and remove excess water, especially if it's not being used i a stew or soup.