Heat a large pan of water on high to boiling for pasta.
While water boils, melt margarine and pour into a casserole dish.
Chop onion and broccoli and place in casserole dish on top of melted margarine.
Add spaghetti to boiling water and cook at boiling until tender (about 8 minutes). Heat may be reduced slightly to prevent boiling over. Turn oven on to 425 degrees.
Drain tuna and layer on top of onion and broccoli in casserole dish.
Drain and rinse spaghetti and add to casserole.
Grate pepper jack cheese and layer on top of spaghetti. Pour milk over the top.
Cover and bake for 30 minutes or until milk has been absorbed. Uncover and continue cooking for another 10 minutes or until all the moisture has been absorbed and the top of the casserole is golden brown.