mk, sed, and embedding environment variables

When using mk , GNU sed, I find that variables do not expand at all. Problematic code: VAR=qux ... build: sed -i "s|FOO = .*|FOO = $VAR|" bar.file This seems to run the following: sed -i "s|FOO = .*|FOO = $VAR|" bar.file when I want it to run sed -i "s|FOO = .*|FOO = qux|" bar.file I have tried using single quotes and double quotes, as other stack exchange posts have mentioned. The only instance I can get $VAR to expand is when there are no quotes around it. Which of course means that sed no longer sees the expression as an expression.