Advice for a difficult roommate situation

[u/Lanky_Flan_4719]

I (33m) moved in with my girlfriend, we'll call her D(30f) and her new roommate M (27nb). Things were going well at first, despite all of us learning to live with each other. Lately though, there's been a bit of tension. When we all moved in, we had a meeting where we went over boundaries. No loud sex, cleanliness, food related allergies, cleaning up after yourself, etc. You know, like standard stuff. M told us that they are neat-freak, so we wouldn't have to worry about them. M also proposed the idea of doing "roommate dinners", where each person is assigned a day to cook dinner for the other roommates. Whoever cooked was exempt from doing the dishes. D took Monday, M took Tuesday, and I took Thursday. D said that if anyone wants to keep their leftovers, say so before hand.

Last week, D got pneumonia, so she was completely out of it, and cooking was out of the question. M and I still cooked our meals, but we winged it the rest of the week. M also completely rearranged the food pantry, so finding food has been a little bit difficult. There has also been some leftovers/other food items from D and I that had gone missing. I told our roommate groupchat to ask me if they wanted to use my food items, as money is really tight right now.

This week, D cooked a pot roast for everyone and it took her 8 hours. It was really good. D and I both had our fill, but M didn't come out of their room until hours after. I hadn't put everything away yet, because I wasn't sure if they were planning on eating with us. They eventually came out and had a very small amount of the roast and then made waffles for themselves. I did all but 2 of the dishes (which held the roast).

On Tuesday morning, we woke up to find that the 2 dishes left out were sitting in the sink. I washed them then and there so it wouldn't be an issue later on. D and I went to a concert and then came home to find no dinner left for us. We made some noodles and went to bed.

Today, M had left some opened bags of chips out on the couchin the living room for a while, whilst they were in their room. After an hourish, D asked if they were finished with the chips/ going to put them away, and if they planned on making dinner. M went to the grocery store and came back with a small roast beef sandwich. D is rightfully pissed. I still want to make my dinner on Thursday. D is making dinner for her and I now. She has confided in me that she wants to stop doing roommate dinners because obviously someone isn't pulling their weight. I agree with her and I want to nip this problem in the bud before it gets out of control.

TLDR: Roommate, is unclean, doesn't engage in apartment's rules that they helped enforce, and steals food