Shouto: udon, saving his brother, and his family's grief
The fact that Shouto's friends didn't respond with disgust or surprise when he expressed regret and sorrow that he doesn't know his villain brother's interests means that there's probably already an understanding and acceptance of Shouto's planning. They've either talked about it or assumed that Shouto wants to not only stop his brother but also have a relationship with him. Iida went out of his way to offer support and empathy because he knows what it feels like to be worried about an older brother because you love them. Not one of Shouto's friends went like, "What?!?!! How could you even think about wanting to get to know Dabi, he murdered people!! He burned Tokoyami and Jirou!"
No. Their reaction was the opposite. Bakugo teases and suggests that Touya's favorite could be udon, which is a reference to Inasa, who tried so hard to form a friendship with Shouto during the remedial lesson chapters. When Inasa said that he was an udon type of guy, Shouto immediately said they were too different to be friends because he likes cold soba and, in his usual casual manner, told him to stop trying to force their friendship.
But, Shouto reacted differently to this hypothetical scenario that Bakugo presented. Shouto basically said he'd be willing to eat udon if it's Touya's favorite food. He wouldn't mind going out of his way to try out whatever Touya liked. Shouto is unfazed at the idea that Touya could be so different from him because that's his brother and he always wanted to get to know him and his other siblings. In the fifth lite novel, he confesses to Natsuo that he always wanted to play with his older siblings, which coincides with the reoccurring panel of a wistful five-year-old Shouto watching them play. He even begged to join them and refused to look away.
People connect and bond through food. At Christmas time, Deku looks around at all his friends eating together and thinks about how happy he is to be there and have them in his life. The Todoroki family was deprived of being able to have bonding time at the dinner table like this. And beyond that, to remember someone’s favorite food is a basic form of intimacy and shows love to someone, which were also fundamental family rites that the Todorokis were robbed of.
This is why Natsuo regrets not knowing Shouto’s favorite food was soba all these years, why he is angry that his efforts to help cook for the family were shot down as not good enough, and why in the fifth lite novel he wants to help Shouto make soba. This is also why Fuyumi took care of not only the cooking but also the house all these years and went above and beyond to invite Shouto's friends to a special dinner. In the first lite novel, Rei offering Shouto what she remembers to be his favorite yogurt as a child was a way of showing that she remembered all of these small details about him. She went out of her way to buy it to make him happy. The fact that she remembered made Shouto both touched and filled with sorrow.
So, it means a lot that Shouto wants to share a meal with Touya. He's looking forward to it. Although it hasn't been explicitly stated, Shouto's plan is clearly to save Touya and bring him home. He didn't struggle to come up with this conclusion like Deku and Ochako are struggling right now. His love - and his family's love - for Touya is unconditional, which is exactly what will save Touya. It's all he's ever wanted: to be seen and wanted.
