Heat the avocado oil in a wide pot over medium-high heat. Add the onion and tomatoes and cook for 7-10 minutes until softened and beginning to char. Season with salt.
Melt the butter into the tomatoes. Add the minced garlic, Thai chili peppers, and white parts of the scallions and cook for 1 minute until fragrant.
Simmer the Sauce:
Add the soy sauce, sesame oil, sugar, and fish sauce. Pour in the chicken stock and bring to a boil. Taste and season to your preferences. Reduce heat to just slightly above low and hard simmer, uncovered, for 20-30 minutes. If the sauce reduces too quickly or too much, turn the heat to very low and add a splash of water.
Prepare the Chili Oil Almonds:
Combine the chili oil and almonds in a small pot and turn the heat to low. Cook in the oil for 20 minutes until the almonds are golden brown. Turn off the heat and stir in the green parts of the scallions. Season lightly with salt and set aside.
Cook the Noodles:
Cook the udon noodles according to package instructions. Drain, rinse, and set aside.
Cook the Shrimp:
Add the shrimp the sauce. Cover and cook for 5-6 minutes until just barely cooked through.
Finish the Udon:
Toss the udon in with the shrimp and continue tossing to coat until the udon is warmed through.
Add 2 tablespoons of the chili oil almonds to the udon and toss to coat. Turn off the heat.
To Serve:
Divide the udon between bowls. Spoon a teaspoon or two of the almonds on to each dish. Reserve any excess chili oil almonds for another use. Enjoy!