When a non-verbal young chef learns that her signature dish has been ripped off by a rival, she must win over a new elite clientele with her intuitive understanding of flavour and micro-dosing.

Hmm.. I like the "reviewed at the end of the month" as it adds a ticking clock (great for loglines). It also gives her a very specific goal. It's good to make sure everything is on the line and the stakes are high. Make sure her restaurant closes down/she loses her job or something if she doesn't come up with the star-worthy dish.

You probably don't need "even if her methods aren't entirely legal" - not for the logline.

So that's...
Intention: must create Michelin star-worthy dish.
Obstacles: ? ex-fiance (who we innately think must be creep and will stop at nothing because he steals signature dishes.)

When her successful restauranteur ex-fiance steals her signature dish, a struggling chef must create a new Michelin star-worthy dish before both restaurants are reviewed at the end of the month.

"Loses her catering job" and "young" are fine for the plot/outline but not sure about for the logline. "Struggling" should suffice.

I like the ex-boyfriend addition. It makes the conflict more personal.

How about:

When a young chef loses a catering job because her ex-fiancé stole her signature dish, she must create a new Michelin star-worthy dish before both their restaurants are reviewed at the end of the month, even if her methods aren’t entirely legal.

When a non-verbal young chef learns that her signature dish has been ripped off by a rival, she must win over a new elite clientele with her intuitive understanding of flavour and micro-dosing.

Intention: win over a new elite clientele.
Obstacle: ?

When a struggling chef finds out her ex-boyfriend has stolen her signature dish and opened a successful restaurant next door, she sets out to beat him at his own game.

(by presumably winning over the new elite clientele with her intuitive understanding of flavour and micro-dosing. Meanwhile - the ex boyfriend can be revealed to be a hack chef/incredible salesman which is the opposite to her non-verbal/perhaps not good with people, etc.

I don’t know if the logline I came up with conveys any of that but these are just ideas.

I made them both restauranteurs to add stakes. Her livelihood is on the line. Perhaps the livelihood of friends/co-workers/family members as well?

Again - these are just ideas to get that ball rolling.

Good luck!