I added some left over marinara sauce before cooking because I thought I needed more liquid. That's probably the culprit.
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Let me guess that this is a Rosh Hashana dish. The only things I can think of would make it totally un-kosher. I have no reason to be observant myself, but I do consider this stuff.
Unkosher is fine with us. Bring it on
You can try the potato trick: add a whole peeled potato & simmer in the gravy. Remove the potato before it begins to fall apart. Taste to see if it's still too spicy, if so, repeat.
Sometimes simmering some cut up potato in the gravy can calm down a too-spicy sauce. You could also add some dairy and/or some sugar. You might have to readjust the seasonings, but another way to counterbalance the spicy marinara is to add a can of unseasoned pureed tomatoes.
If it's not to be a kosher thing I would try a scoop of sour cream. But. I never know what too spicy really means to people ...believe me I deal with this all the time.
I second the sour cream option if you're going unkosher... If you don't want the dairy, then I'd up the volume of gravy which would in turn lessen the volume of spice overall... Obviously this may up it to vast volumes of gravy! Some extra stock, a tin of toms, some other spices to complement the heat, and a bit of cornstarch to thicken if it's needed, and no one but us need know of the misadventure.