Referencing Env variables with a variable in a Jenkinsfile

EnvInJenkinsfile.groovy

// I wrote a Jenkinsfile shared library method to check job parameters for mandatory fields. The 
// list of mandatory fields depended on other parameter values. Writing this as a reference because 
// it wasn't immediately obvious how to use a (groovy) variable to access an env variable.

properties([
    parameters([
        string(name: 'MY_PARAM', defaultValue: '1.0.0', description: 'Useful description here.'),
    ])
])

node{
    echo MY_PARAM // Prints '1.0.0' assuming the param is left as the default value
    def mylist = ['MY_PARAM']
    mylist?.each {
        echo it // Prints 'MY_PARAM'
        // echo $it // Error, no such property $it 
        echo 'env.${it}' // Prints 'env.${it}', single quotes do no variable interpolation
        echo "env.${it}" // Prints 'env.MY_PARAM', variable interpolation on it but not for env
        echo "$env.MY_PARAM" // Prints '1.0.0'
        echo "$env.it" // Prints 'null'
        echo "$env.${it}" // Prints 'org.jenkinsci.plugins.workflow.cps.EnvActionImpl@4ba0a01e.MY_PARAM'
      
        // Solution
        def myvar = env[it]
        echo myvar // Prints '1.0.0'
    }

}