I'm only thinking this bc I just literally did this for my cats, but radioapple fic where Alastor is NOT a morning person.
Like even remotely. At all. (He stays up late for his radio broadcasts - you think he's out of bed before 10am unless he HAS to be? No.)
However. He's also the one who makes everyone their breakfast because he's the only competent chef at the hotel. (There's a reason before he showed up all Angel Dust ate was popsicles, Charlie is sweet but her attempts to cook could be considered lethal.)
So he drags his ass downstairs at like 6am to make everyone breakfast (and then utilizes his fire/shadows to keep the food warm for everyone to eat whenever they get up.) And then once he's done he goes back to bed for a few hours before eating his OWN breakfast. (A nice, tasteful raw venison, instead.)
But he absolutely doesn't bother getting even remotely dressed or even brushing his hair. Just big poofy and sleep mussed hair, rumpled pajamas, and he's yawning every few minutes. This man is half awake and is working on instinct at this point.
Lucifer had absolutely no idea about this until one day his insomnia gets the better of him, and he's also awake at 6am, and he hears someone in the kitchen, so he decides to explore.
And he doesn't even know how to react to soft fluffy Alastor calmly making everyone breakfast, seeing his cute little deer tail just casually out, his hair not even brushed, and wearing significantly less layers than normal.
(and if you really want to give him a heart attack, make Alastor not sleep in long full pajamas but instead just shorts and a slightly oversized t-shirt that hangs off one shoulder - make Lucifer come to terms with his long ass deer legs, his cute little hoofies, AND his collarbones. All at 6am.)
But to top it all off, Alastor is just too tired and half-asleep to even react to Lucifer showing up. All Alastor does is ask if he wants anything different for breakfast, coffee, or something. And Lucifer is too confused to do anything other than agree, and he's not sure how to handle the dude who, the day before, was at his throat calmly handing him a plate of breakfast and a coffee. What.
And then Alastor, in between yawns, is like I'm going back to bed. And just leaves.
"You're not eating any of this????!" - "Hah! No."
Lucifer is suspicious, so he discreetly follows Alastor back to his room, only to see him faceplant into his bed and fall back asleep. (That's kind of cute. Wait, what? No, it's not!)
(When he re-emerges at 11am, he's just as much of an asshole to Lucifer as he was the day before. 6am Alastor and 11am Alastor are two different people.)
Lucifer may decide to become a morning person just to keep seeing this version of Alastor. Who knows.