I have a question about the recipe "Pastitsio" from TheRunawaySpoon.
You can definitely replace the lamb with beef but I'd probably stick to the tomato.
Those substitutions would really change the flavor of the dish, giving it more of an Italian taste than the unique Greek flavor. Substituting beef isn't such a big deal, but using marinara suce would take it in a really different direction.
I've made this with both beef and lamb, and IMO lamb is tastier, but beef is good. I was told by a woman of Greek heritage that it's the best pastitsio she's ever had. Stick with the tomatoes!