Put celery, pepper and onion into a food processor and pulse until chopped.

Heat oil in a large pot over medium-high heat, add vegetables and cook, stirring occasionally, until golden brown, 6 to 8 minutes.

Add flour and cook, stirring, until moistened, about 1 minute.

Add stock and cook, scraping up any browned bits, until reduced by two-thirds, about 4 minutes.

Whisk in soymilk.

Stir in potatoes, thyme, salt and pepper, and bring just to a boil.

Reduce heat to medium-low, partially cover, and simmer until fragrant and thickened, and potatoes are almost tender, about 15 minutes.

Stir in monkfish and corn and cook until fish is just cooked through, 6 to 8 minutes more.

Stir in parsley and serve, garnished with celery leaves.