Heat 1 Tbsp oil in a large pan over high heat until very hot.
Cut roast into 2-inch chunks and season with salt and pepper.
Add half of the meat to the pan, searing for 1-2 minutes, until nicely browned, before flipping or stirring.
Sear for another 2 minutes and use a slotted spoon to transfer meat to the crock.
Repeat for remaining meat.
Add onion, potatoes, carrots to the crock pot.
Add the chicken broth, wine, and tomato paste (I dropped it in by the spoonful and let it distribute while cooking).
Top with garlic and herbs.
Cook on low for ~8 hours.
At the end of the cooking time, remove 1/4 cup cooking liquid and whisk in cornstarch until smooth.
Stir slurry into the crockpot, cover, and let cook for ~20 minutes.
Stir in the spinach and basil and serve.