Depends on your cultural preferences and what you have on hand - and how much you like cooking.
When I have an OMAD day (usually I do TMAD to keep my protein up as I do strength training) I eat a huge salad or servings of low carb veg that take up 2/3 of my plate and provide lots of alkalinizing antioxidants (the keto-green approach), plus aim for around 30-40g protein, cooked in good fat - with a side of extra crunchies like almond flour crackers or flax seed crackers. Sometimes that means a huge salmon/chicken/tuna salad bowl with lettuces & crackers - other times an Italian bowl with cheeses, salami or other meat, olives, artichokes, and some sundried tomato. Other times it might be leftover Indian restaurant Saag/Palak green mush with eggs and cottage cheese.
Figure the fatty protein serving you’re aiming for, then round it out from there. The classic LCHF book Protein Power has lots of helpful menu plans in general for that, though not OMAD oriented it’s quite informative.
The more you have on hand in your house, the easier it gets to combine for tasty OMADs without stress. Personally, I’ve always preferred to have at least a bit of color on my plate for VLC mostly meat meals - which means some sauteed long green onions, or a bit of fried tomato, and/or paprika sprinkles - variety is the spice of life as they say.
The classic LCHF/keto books have lots of great recipes to explore - The New Atkins DIet book by Westment/Phinney/Volek also emphasizes daily veg and has a great restaurant guide for doing keto with a variety of cultural cuisines - and also George Stella’s Quick & Easy Keto cookbook is great for ideas that are easy to make and gourmet-level taste.
I also like to work with canned organic black soybeans when I’m making a Latin American type of OMAD, they’re super low carb and are great fried and eaten with sour cream with other animal protein and veg and hot sacue/salsa to make a rather exciting platter of food including crispy heated kale wraps or flackers for added crunch.
It’s also good to either add a little chopped organ meats to your cooked proteins, or supplement with high quality beef liver capsules or B complex to round out the micronutrients.